I managed to duplicate the bug reliably on firefox : go on a page where
you must enter stored credential. As I have a master password, I get the
first popup to ask for it and when it prompt for the second popup with
the credential themselves, it crashes.
In TB as, I have tree popups (bug as it ask for the master password tree
times) for different email accounts, it crashes also when opening the
second one, once the main TB window is open (the first one appears
before the main window in my case).