Firefox trunk nightly on Mac OS X not staying set as default browser?
If you are using nighties of trunk on Mac OS X and have found that your default browser choice is reverting to Safari, you are hitting bug 554997.
The bug is "fallout" from bug 553073. It should not affect any release version of Firefox and should only affect testers who auto-updated from a build without bug 553073.
There is a simple workaround. In Terminal, run:
touch /Applications/Minefield.app
Of course, change the location to where you actually have the nightly if it isn't in /Applications.
If you are not comfortable with the Terminal, simply moving the application to the desktop and back should fix the issue as well.
As a consequence of fixing bug 553073, the default browser choice should now stick if you run Firefox and Minefield side by side.