Bug 588909 is now in Firefox Beta. This replaces the 20100101 date in the Firefox UA string with a version number. The date has been frozen for a long time and doesn’t give any useful information. Gecko versions and Firefox versions are now aligned, so it makes sense to have the UA string reflect that (and match what we are doing with Firefox on Android and B2G).
However, despite the date not telling people anything, some sites do seem to be detecting it (see my previous blog post). We have already deferred this change a few times as we found and evangelised sites. We can contact broken sites to get them to fix it (it should be an easy fix) and, if contacting them fails, we do have a UA string override mechanism.
If you see any sites starting to reject Firefox Beta (i.e. 17) as an unknown browser, please file evangelism bugs and then follow them up by contacting the site. You can use User Agent Switcher to check whether the problem outlined here is, in fact, what’s wrong. Thanks :-)