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Mac OS - Mozilla Registry in Preferences under System Folder. If present, also delete Mozilla Versions.
UNIX - registry in ~/.mozilla/
Windows - mozregistry.dat, and mozver.dat if it is present, in Windows or Winnt.
If you intent on using Mozilla frequently, and especially update to newer builds once every few days, it would be wise to back up files important to your Mozilla profile. Especially, keep an extra and updated copy of your bookmarks file, and take other precautions to avoid lossing important information! Although Mozilla is stable enough to use on a regular basis, switching to newer builds might bring in significant problems with that build. For example, there was once a bug that caused the entire bookmarks file to be deleted, forcing developers to remove that build to from the server (the only instance that I remember a build being removed), and a new build was quickly released with this problem resolved. Although this was a very rare instance of a very damaging bug, it was unexpected for those who were unfortunate to download that build before it was removed from the server.