You can remove Firefox 3 through the Control Panel in the Start Menu on Windows, by removing the Firefox application on OS X, or by removing the firefox folder on Linux.

Removing Firefox 3 won’t remove your bookmarks, web browsing history, extensions or other add-ons. This data is stored in your profile folder, which is located in one of the following locations depending on your operating system:

Windows Vista Users\<UserName>\AppData\Roaming\Mozilla\Firefox Windows 2000, XP, Server 2003 Documents and Settings\<UserName>\Application Data\Mozilla\Firefox Mac OS X ~/Library/Application Support/Firefox Linux and Unix systems ~/.mozilla/firefox

Any version of Firefox that you install after removing Firefox 3 will continue to use the data from this profile folder.