For whatever reason (I can only assume it has to do with the product being released /after/ Ubuntu 9.04 came out) the Ubuntu devs have decided to call Firefox 3.5 “Shiretoko”. You can install alongside your regular Firefox installation by typing aptitude install firefox-3.5 and you can run it by typing firefox-3.5. That’s all well and good, except that the User-Agent string is set to:

Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.2) Gecko/20090803 Ubuntu/9.04 (jaunty) Shiretoko/3.5.2

instead of:

Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.2) Gecko/20090803 Ubuntu/9.04 (jaunty) Firefox/3.5.2

Which breaks sites like Facebook (Facebook chat doesn’t work). To fix this, I installed the “User Agent Switcher” extension, which, after a few days of use, I have decided I don’t like. It doesn’t remember user preferences, and you have to reset your default User-Agent every time you start Firefox.

Looking around on the web, I found a much easier way to solve this problem. Type “about:config” in your address bar, and search for the “general.useragent.extra.firefox” entry, right click on it, choose “modify” and change “Shiretoko” to “Firefox”.