GPGee (website down during time of writing) causes Thunderbird to crash on opening mails containing http:// links after having opened a mail containing a file:// link before. This sentence alone should make it clear to everyone how much trouble this issue caused me, and how hard it was to track down… Well – at least I learned a thing or two about debugging (Mozilla applications) on Windows.
See my bug report in Mozilla’s bug tracker if you want to read the whole story. You can even watch a screen video of the crash there 😉