Application data in Firefox is used to house your Mozilla Profile.  The Profile used in Firefox stores your preferences, extensions and themes.  It also coordinates saved passwords, your cache and your cookies.  Without a Profile saved in Firefox, you would have to start over every single time from scratch.  These Profiles allow you and other users to personalize Firefox in ways other browsers cannot be customized.  It makes Firefox a unique Internet browser.

Your first step to importing application data is to copy and paste the Profiles folder from the Firefox hidden data folder. In order to do this, you have to open any folder, go to Tools and Folder Options. Once in Folder Options, click on the View tab and check Show hidden folders and files. This will make the application data folder viewable to you. To get to it, click on My Computer and then head to your C:\ and the Documents and Settings folder.

Enter your login name folder and find the Mozilla or Firefox folder, once inside copy and paste the Profiles folder to the desktop. Once you have uninstalled and reinstalled Firefox, simply copy and paste that Profiles folder over the new one the installation creates. Click yes to overwrite it and then open Firefox to begin browsing with your old Profiles intact.

If your application data fails to load in Firefox, there may be a conflict of information.  Perhaps, you are installing older application data than the version of Firefox that was reinstalled.  You may also have a conflict of extensions, themes or add-ons that are causing issues.  If this is the case, you may have to start a fresh Firefox Profile.  In order to do that, you simply delete the Profiles from Firefox and open the browser to create a new one.  While this may not be the ideal situation, the process does not corrupt your browser installation.