I had a look at the defects already logged in firefox web site, and non seem to look like the problem we've got.
There's chances this is actually not a Firefox problem but something related to the add-ons or extensions.
Have you figured which particular usage pattern triggers the problem? Because I haven't.
The next step is to use Firefox and Thunderbird in "safe mode" and see if the problem still appears.
To run Firefox in safe mode: launch whichever equivalent of Krunner you have in xfce (Alt-F2) and type "firefox -safe-mode".
Let's see what happens...
More about the safe mode here: https://support.mozilla.org/en-US/kb/tr ... -safe-mode