Firefox 5 has evolved to a new sneaky way... the new default setting will just update users' Firefox browser to whichever new version they deem fit, without even allowing users delay the update slightly, if the end users have not messed around the preference settings. Only a brief message that the Firefox is updating itself is shown.
Understandably, many users believe new versions (from Mozilla) are good, and most of them do not mind being rushed to the latest version. Just because Mozilla's forced self-updating default setting is under the excuse of addressing security and stability issues, many people (particularly those open source extremists) do not mind effectively being rushed to update. It is also well understandably that many of those very same people do not extend such updating style to Microsoft and Apple. In the case of Firefox 3.6.19, such excuse has been proven to be completely untrue.
UserAgent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:126.96.36.199) Gecko/20110803 AlexaToolbar/alxf-2.14 Firefox/3.6.20