Daniel Lange's blog

If you see Thunderbird hanging during startup for a minute and then continuing to load fine, you are probably running into an issue similar to what I saw when Debian migrated Icedove back to the "official" Mozilla Thunderbird branding and changed ~/.icedove to ~/.thunderbird in the process (one symlinked to the other).

Looking at the console log (=start Thunderbird from a terminal so you see its messages), I got:

console.log: foxclocks.bootstrap._loadIntoWindow(): got xul-overlay-merged - waiting for overlay-loaded
[.. one minute delay ..]
console.log: foxclocks.bootstrap._windowListener(): got window load chrome://global/content/commonDialog.xul

Stracing confirms it hangs because Thunderbird loops waiting for a FUTEX until that apparently gets kicked by a XUL core timeout.
(Thanks for defensive programming folks!)

So in my case uninstalling the Add-On Foxclocks easily solved the problem.

I assume other Thunderbird Add-Ons may cause the same issue, hence the more generic description above.