pillmuncher It's not actually a Mozilla bug. We have to set some environmental variables in the Firefox launch script in order to make Firefox work correctly on Solus. The problem is that these environmental variables are also used by Thunderbird since they share much of the same code and if they're set to the values FF needs it breaks Thunderbird. When you click on a link in FF that opens something registered to be handled by TB (like mail links or calendar invites) it opens TB as a child process of FF and TB inherits the environmental variables from FF (which breaks it).
Ideally there would be TB and FF specific versions of these variables so that they wouldn't conflict anymore, but as far as I can tell that isn't implemented yet upstream. I'll look into creating a patch to add them.