This is completely independent of the Edit in TextMate feature, but I've been experimenting with using Firefox lately and I've discovered a nice extension called mozex. I'm using the development version 1.9.x and it allows you to easily configure TextMate as the editor for TextFields. Another nice feature is that mozex lets you configure TextMate as your page source viewer.
This is actually old news :)
http://macromates.com/wiki/Main/Howtos#Firefox
We should make all the info more accessible somehow.
Soryu.