The version information you see in the About dialog is not directly available to websites. Instead, they see your "useragent" text, which can be modified. In your case, it has been modified to list version 3.5.4. This sometimes is caused by an add-on. Although this article is a bit old now, the procedure it describes still is the best one I know of for fixing this problem: [https://support.mozilla.org/en-US/kb/Websites%20or%20add-ons%20incorrectly%20report%20incompatible%20browser Websites or add-ons incorrectly report incompatible browser | Troubleshooting | Firefox Help]. Any luck?