Daniel Lange's blog

Let's run:

Now browsing through the resulting /var/tmp/trace you'll find:

[...]
... this is the downloaded file where context menu (right click) -> "Open Containing Folder" has been clicked:
stat("/home/dlange/Downloads/vpngate_jawws.opengw.net_udp_1194.ovpn", {st_mode=S_IFREG|0664, st_size=8702, ...}) = 0
... and now Firefox goes to search and find which application to open a directory with:
stat("/home/dlange/.local/share/applications/mimeinfo.cache", 0x7fff4e864400) = -1 ENOENT (No such file or directory)
stat("/home/dlange/.local/share/applications/mimeinfo.cache", 0x7fff4e8644e0) = -1 ENOENT (No such file or directory)
stat("/home/dlange/.local/share/applications/defaults.list", 0x7fff4e864420) = -1 ENOENT (No such file or directory)
stat("/home/dlange/.local/share/applications/defaults.list", 0x7fff4e8644e0) = -1 ENOENT (No such file or directory)
stat("/home/dlange/.local/share/applications/mimeapps.list", {st_mode=S_IFREG|0600, st_size=644, ...}) = 0
stat("/usr/share/applications/mimeinfo.cache", {st_mode=S_IFREG|0644, st_size=35278, ...}) = 0
stat("/usr/share/applications/defaults.list", {st_mode=S_IFREG|0644, st_size=9976, ...}) = 0
stat("/usr/share/applications/mimeapps.list", 0x7fff4e864420) = -1 ENOENT (No such file or directory)
stat("/usr/share/applications/mimeapps.list", 0x7fff4e8644e0) = -1 ENOENT (No such file or directory)
stat("/usr/share/kde-plasma/applications/mimeinfo.cache", 0x7fff4e864400) = -1 ENOENT (No such file or directory)
stat("/usr/share/kde-plasma/applications/mimeinfo.cache", 0x7fff4e8644e0) = -1 ENOENT (No such file or directory)
stat("/usr/share/kde-plasma/applications/defaults.list", 0x7fff4e864420) = -1 ENOENT (No such file or directory)
stat("/usr/share/kde-plasma/applications/defaults.list", 0x7fff4e8644e0) = -1 ENOENT (No such file or directory)
stat("/usr/share/kde-plasma/applications/mimeapps.list", 0x7fff4e864420) = -1 ENOENT (No such file or directory)
stat("/usr/share/kde-plasma/applications/mimeapps.list", 0x7fff4e8644e0) = -1 ENOENT (No such file or directory)
stat("/usr/local/share/applications/mimeinfo.cache", 0x7fff4e864400) = -1 ENOENT (No such file or directory)
stat("/usr/local/share/applications/mimeinfo.cache", 0x7fff4e8644e0) = -1 ENOENT (No such file or directory)
stat("/usr/local/share/applications/defaults.list", 0x7fff4e864420) = -1 ENOENT (No such file or directory)
stat("/usr/local/share/applications/defaults.list", 0x7fff4e8644e0) = -1 ENOENT (No such file or directory)
stat("/usr/local/share/applications/mimeapps.list", 0x7fff4e864420) = -1 ENOENT (No such file or directory)
stat("/usr/local/share/applications/mimeapps.list", 0x7fff4e8644e0) = -1 ENOENT (No such file or directory)
stat("/usr/share/applications/mimeinfo.cache", {st_mode=S_IFREG|0644, st_size=35278, ...}) = 0
stat("/usr/share/applications/defaults.list", {st_mode=S_IFREG|0644, st_size=9976, ...}) = 0
stat("/usr/share/applications/mimeapps.list", 0x7fff4e864420) = -1 ENOENT (No such file or directory)
stat("/usr/share/applications/mimeapps.list", 0x7fff4e8644e0) = -1 ENOENT (No such file or directory)
... the various mimetype caches are done, now for the individual .desktop files:
open("/home/dlange/.local/share/applications/nautilus-folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/home/dlange/.local/share/applications/nautilus/folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/home/dlange/.local/share/applications/nautilus-folder/handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/nautilus-folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/nautilus/folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/nautilus-folder/handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/kde-plasma/applications/nautilus-folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/kde-plasma/applications/nautilus/folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/kde-plasma/applications/nautilus-folder/handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/share/applications/nautilus-folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/share/applications/nautilus/folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/share/applications/nautilus-folder/handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/nautilus-folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/nautilus/folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/nautilus-folder/handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/home/dlange/.local/share/applications/dolphin.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/dolphin.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/kde-plasma/applications/dolphin.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/share/applications/dolphin.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/dolphin.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/home/dlange/.local/share/applications/nautilus-folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/home/dlange/.local/share/applications/nautilus/folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/home/dlange/.local/share/applications/nautilus-folder/handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/nautilus-folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/nautilus/folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/nautilus-folder/handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/kde-plasma/applications/nautilus-folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/kde-plasma/applications/nautilus/folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/kde-plasma/applications/nautilus-folder/handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/share/applications/nautilus-folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/share/applications/nautilus/folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/local/share/applications/nautilus-folder/handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/nautilus-folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/nautilus/folder-handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/nautilus-folder/handler.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/home/dlange/.local/share/applications/rawtherapee.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/applications/rawtherapee.desktop", O_RDONLY) = 44
... o.k., it found a .desktop file to read
fstat(44, {st_mode=S_IFREG|0644, st_size=1101, ...}) = 0
read(44, "[Desktop Entry]\nType=Application"..., 4096) = 1101
read(44, "", 4096)                      = 0
close(44)                               = 0
access("/usr/local/sbin/rawtherapee", X_OK) = -1 ENOENT (No such file or directory)
access("/usr/local/bin/rawtherapee", X_OK) = -1 ENOENT (No such file or directory)
access("/usr/sbin/rawtherapee", X_OK)   = -1 ENOENT (No such file or directory)
access("/usr/bin/rawtherapee", X_OK)    = 0
... Firefox goes on to start /usr/bin/rawtherapee
[...]

You easily see the massive amount of locations Firefox tries to find a suitable mime entry. If you look carefully, you can also spot the issue.

open("/usr/share/applications/dolphin.desktop", O_RDONLY) = -1 ENOENT (No such file or directory)

indicates failure, the dolphin.desktop file was not found. Wait a moment, I use dolphin every day. It's there. But it's installed in /usr/share/applications/kde4/dolphin.desktop. Ouch! So Firefox does not search the applications-dir recursively nor does it have applications/kde4 in its search path for .desktop files.

So a solution is easy to implement now. We need to make sure Firefox finds the dolphin.desktopfile before it finds the rawtherapee.desktop file:

Now the dolphin.desktop file will be found earlier in the process and Firefox will open the Downloads folder in Dolphin again.

But why?

The underlying issue revealed here is one that is very common in loosely coupled cooperation situations.
People rely on an assumption, on an interpretation of a (vaguely formulated) standard, on what's (often wrongly) thought to be best practices.
And the differences in interpretation manifest themselves right into a breakage.

Firefox is developed as a Gnome application. The developers adhere to the Freedesktop standards, e.g. the one covering .desktop files. Now sadly so, this document does not describe what to do with the .desktop files, it describes the what but not the how.

Apparently the Firefox developers assumed reading /usr/share/applications/*.desktop (and a few other locations like that, see the strace dump above) was good enough. They wanted to save the effort of recursing all underlying directories which is a potentially very slow (expensive) process. The Gnome documentation on desktop files does not indicate the directories need to be recursively searched. It reads "Place [the .desktop] file in the /usr/share/applications directory so that it is accessible by everyone, or in ~/.local/share/applications if you only wish to make it accessible to a single user." So it's easy to assume placing and searching files in the top-level directory is sufficient.

Now the KDE folks lead the definition of the Freedesktop Menu specification and that one even gives an example for their friends from Gnome: "GNOME applications [install] desktop files [...] in datadir/applications/ or it could install desktop files in a datadir/applications/gnome subdirectory."

The KDE folks assumed that the joint Freedesktop proposal was sufficient to use a clean /usr/share/applications/kde4/ to store their (in my case 151) .desktop files from all KDE apps as everybody would be recursing the applications directory now.

The Menu specification from Freedesktop states: "If the directory contains sub-directories then these sub-directories should be (recursively) scanned as well."

Well, should is not must. RFC2119 defines:

MUST

This word, or the terms "REQUIRED" or "SHALL", mean that the definition is an absolute requirement of the specification.

SHOULD

This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.

So this is a case of conflicting assumptions facilitated by an insufficiently strict standard.

Update:

02.06.2013: a user in ##linux on freenode reported the same issue with audacious. Here again the mimetype inode/directory claimed by audacious is the culprit. In this case just copying Thunar-folder-handler.desktop to ~/.local/share/applications/ and running update-desktop-database ~./local/share/applications/ solved the issue.

21.07.2014: Recently somehow an application/octet-stream=bluefish.desktop; ended up in my desktop's ~/.local/share/applications/mimeapps.list. Probably because I told Firefox to open some other file that was served as application/octet-stream (but was a text file by content and file extension) in Bluefish. Apparently it does not save that connection bound to the file extension but bound to the (wrong and generic) mime-type. This in turn leads to Firefox wanting to open .pdf files downloaded from the net with that generic mime-type in ... Bluefish now. Removing the application/octet-stream=bluefish.desktop; line from the mimeapps.list file solves that issue.