It seems you are not alone with this, some users are actually running FF3 but it still shows as FF2.
If this is what is happening to you then one site reccommends this:
Your user agent string is corrupted (identifies the version of Firefox).
To reset your user agent string.
Type about:config into the location bar and press enter. This brings up a list of preferences. Enter general.useragent into the filter box, this should show a few preferences (probably 4 of them). If any have the status user set, right-click on the preference and choose Reset