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.
There is a simple workaround. In Terminal, run:
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.