When you upgrade it will use your existing bookmarks, history, cookies, passwords and other user data.

During the upgrade it will check your add-ons to see if they are compatible with Firefox 4. If they are not compatible it will check to see if updates are available, and if no updates are available the add-on will be disabled.

The following link lists most of the popular add-ons hosted on the Mozilla add-ons site and shows the maximum version of Firefox they work with. Those listed as 4.0.* or higher should work on Firefox 4.

https://addons.mozilla.org/en-US/firefox/compatibility/report

See also:

• http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox
• http://kb.mozillazine.org/Profile_backup

You need to check these links to know more..

• https://addons.mozilla.org/en-US/firefox/compatibility/report
• http://kb.mozillazine.org/Profile_backup