I've been running the Mozilla Nightly builds of Firefox for months and found a simple way to make everything work, without having to use CLI for anything. it will also leave the old Firefox install in place, just in case something goes wrong. I don't use any add-ons or extensions with Firefox, so I don't lose anything by running these builds.
I grab the tar.bz2 file from here - http://ftp.mozilla.org/pub/mozilla.org/ ... est-trunk/
I then close Firefox.
Click Menu > Home Folder > File System > right-click the opt folder and select Open as administrator.
Right-click the firefox folder and rename it to firefox-old.
Drag the tar.bz2 into the opt folder, right-click it and select Extract Here.
Once the new firefox folder has been set up, delete the tar.bz2 file from opt.
I can then open the latest Nightly build of Mozilla Firefox.
I do the same thing with the EarlyBird versions of Thunderbird, which can be downloaded here - http://ftp.mozilla.org/pub/mozilla.org/ ... earlybird/
If I open the latest build and there are issues, it is as simple as deleting the build and renaming the folders, to get the previous versions working. Every now and again, I do find a Nightly build that is buggy, but I just report the bugs, revert to an earlier version and drive on for another 24 hours, until the next build becomes available.