Firefox and Thunderbird together

One of the most frequently asked questions is "why doesn't Thunderbird start when I have Firefox open?" (you can exchange Firefox and Thunderbird in the question).
The answer is that you should have installed a build of both Firefox and Thunderbird that use exactly the same DLLs, otherwise you most likely won't be able to use them together. What this really means is that the author of the port should have compiled the packages roughly at the same time, using sources that are at the same level.
Obviously, this is not something you are in control of, unless you're able to do your own port. And sometimes you may have a reason to use applications at different levels.
The most straightforward solution is to use the RUN! package by Rich Walsh, which sets up all the environment variables needed to achieve your goal.
You simply copy the run!.exe program to the directory where thunderbird.exe resides, and rename it to something like thunderbird!lk.exe. You then change your program object to point to thunderbird!lk.exe instead of the plain tunderbird.exe and you're done.
You can apply this fix to firefox.exe too, just remember you don't need to apply it to BOTH: just one of them is enough.