'''If you are having a specific issue with Firefox 5, please post the details, and someone on this support forum will try to help you.''' With Mozilla's accelerated release plan, the "plan" is for 5.0 to replace 4.0.1, with no further support for 4.0.* versions. *This will also be the case with versions 6 and 7, currently scheduled for release later this year. *As new versions are released, support for the prior version will be dropped. *The only exception, as of right now, is that version 3.6.x will continue to be supported for 3-4 months, as of this posting. No significant user-interface changes occurred between Firefox 4 and Firefox 5. 95% of the Add-ons hosted at the official Add-ons website are compatible with Firefox 5.0 - https://addons.mozilla.org/en-US/firefox/compatibility/report For extensions that are hosted elsewhere, you can try using this extension - https://addons.mozilla.org/en-US/firefox/addon/add-on-compatibility-reporter/ - to force "incompatible" extensions to be enabled in 5.0 and see for yourself if they work. Then there's a "compatibility" button in the Add-ons Manager tab to report if each extension is working properly or to tell the developer of each extension what problems you're having.