Okay, now it works... to an extent.
Let's say I have Thunderbird open, and I have Firefox open.
If I click a link in an email, since Firefox is open, it will ask me which Firefox profile I want to use. If I select 'default', which is the only profile I have on the machine, then I will get an error that it is already in use. If I close Firefox, then all is well.
On the other hand, we have this...
If I have Thunderbird and Firefox open already, and click a mailto link on a webpage, it will ask me which Thunderbird profile I want to use. If I select default, once again, the only one on the machine, then I will get an error that the profile is already in use. If I close Thunderbird, all is well. I cannot figure out, for the life of me, why they made this so damn difficult. I thought one of the 'selling' points of the Mozilla project was how well Thunderbird and Firefox played together... Oh well... If I was a programmer, I would be able to actually help instead of gripe