Over at Mozilla, in bug 465822 (https://bugzilla.mozilla.org/show_bug.cgi?id=465822), a problem occurred with the jQuery browser detection and the following User Agent, as used by Fedora Linux:
Mozilla/5.0 (X11; U; Linux i686; en-GB; rv:220.127.116.11)
Gecko/2008111217 Fedora/3.0.4-1.fc9 Firefox/3.0.4
$.browser.version returns "3.0.4" here. Whereas with an official Mozilla build and a user agent such as:
Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:18.104.22.168) Gecko/2008102920 Firefox/3.0.4
it returns 22.214.171.124 (i.e., the Gecko version).
This needs to be made consistent: I would even expect to always get 3.0.4 when asking for the browser version, but the way you do it is up to you. In any case, these two browser instances should return the same value for $.browser.version, so they can be detected correctly.