When did you update it? If it was BEFORE June 30 then it certainly wouldn't be 3.5 it would be 3.11
With Beta (test) versions of anything they generally are not offered and a regular update, you have to opt to try it out.
To check for sure in Firefox go up and click Help, About Firefox. It will show you the version number. If it says 3.5 beta there then it's the beta version. Look in Add/Remove. Click on Mozilla Firefox and it should tell you the date you installed it. A lot of people are having problems with this new version. Many of the add-ins have not updated yet so are unusable or don't work properly