While converting my old “Your browser is out of date” messaging on this site from using a page redirect to an in-page message div, I realized that I had messed up my own browser detection for the latest released versions of Gecko-based browsers. doh!
It just goes to show that detecting vendor/version is problematic and should be avoided unless you have a very good reason.
If you are interested in the code, you can check it out. Let me know if you find any errors in this version.