Since I do extensive testing with multiple builds of Firefox, I have several profiles. The biggest hassle when creating a new profile is you start with a clean slate.  That is you start with no bookmarks, extensions, themes, saved passwords or form history, permissions, browser history, etc. However most of this information can easily be migrated to another profile if you know what files you need. Simply copy the files below from the folder of your main profile to that of the new profile(s):

• Bookmarks: bookmarks.html
• Browser History: history.dat
• Passwords: singson.txt and key3.db
• Saved Forms: formhistory.dat
• Site Permissions: hostperm.1

While it is possible to move your userChrome.css, userConent.css, user.js and prefs.js files to the new profile it is strongly recommended you DO NOT do this. Problems with corrupted, damaged, malfunctioning and/or misbehaving profiles are usually caused by one or more of these four files. Moving these files over to a new profile is going bring the same problems to your new profile.

You will still need to reinstall your extensions and themes but at least you will have everything else migrated over fairly quickly and painlessly. For more information about migrating profiles see link below.

Information Source: mozillaZine Knowledge Base ~ Migrating Profiles