Your user agent shows an old 4.0 beta 1 version:

Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:2.0b1) Gecko/20100630 Firefox/4.0b1

You can get the latest beta here: http://www.mozilla.com/firefox/all-beta.html

See also http://kb.mozillazine.org/Testing_pre-release_versions

I was also looking for the answer to this question and cor-el's response is anything but helpful. If you'd like to see your current version, then all you need to do is go to "Help/About". If you'd like to know what build you are using, the only thing that I can find is by going to "Help/Troubleshooting/about:buildconfig/Built from [link]". There you will find some cryptic message which may or may not involve your current build. This is an terribly convoluted way to present information to the end user when all they needed to do was to simply put this information in the About window next to the current version!

Dysmorphophobic, the reply was indeed helpful at the time as his post was from almost a year ago when they were still putting the Beta version on end of the Firefox user agent.

Unlike the Beta builds of before Firefox 5.0, the Beta builds now are much more like the Release Candidates of before which did not have any build version any where (though one could still tell by build date then) including in About window.