See: * http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox * [[Recovering important data from an old profile]]

I didn't read the looooooooooooog stories above but have found the simple solution by myself and hereby share below. '''Please note that the steps below are aim to ''MOVE'' and ''OVERWRITE'' the profile (contains bookmarks & extensionse, etc.) from FF4 beta to FF3.x.''' Steps: 1. !!! BACKUP !!! - Backup everything (including the folder ''[random characters].default'' itself) in your FF3.x profile which is likely located at ''C:\Documents and Settings\[your user name]\Application Data\Mozilla\Firefox\Profiles\[random characters].default\'' 2 - Upgrade the old FF3.x to 4.0 final 3 - Ignore any extension compatibility warnings.. and Close FF 4 - Delete everything inside the profile folder ''[random characters].default'' 5 - Find the FF4 beta profile, the path is ''different for everyone''! My path is like this: ''C:\Test_Programs\FirefoxPortableTest\Data\profile'' 6 - Copy everything inside the FF4 profile ''(..\Data\profile\)'' folder to the old profile folder ''(..\[random characters].default\)'' 7 - Finished. If you feel regret after such an operation... you know now why step 1 is important.. huhh?