You should know that Firefox 8 automatically disables all third-party add-ons by default and user installed ones will be kept, after installing the latest Firefox beta release users will be presented with one-time dialog to select the add-ons which they wish to keep or disabled. Mozilla recommends users to disable unnecessary add-ons to improve Firefox performance.

 add-on selection dialog firefox 8

You can always manage or enable disabled add-ons by Firefox from Add-ons Manager, but this one-time notification for you is to know all third-party add-ons are disabled, you can enable some or all of them if you wish to keep.