[See update below.] The Mozilla foundation released Firefox 18.104.22.168 today, in order to pre-emptively fix some security problems and improve compatibility with Windows Vista. But 100% support of Vista is proving to be more difficult than originally imagined. For example, one user on a community thread in January 2006 optimistically wrote:
Firefox doesn't have to do anything. It runs just fine on its own, whether it be on Windows 98, 2000, XP, Vista, or whatever. Just because Microsoft comes out with a newer version of Windows has nothing to do with Firefox or any other application suddenly "breaking" and requiring an update for support.
However as more people started using Vista it became clear that Microsoft had made a number of changes, many in order to make Internet Explorer 7 more secure on Vista, and that some of those changes broke Firefox. In August, the Firefox team even accepted some help from Microsoft to get these issues addressed but problems remain.
Version 22.214.171.124 lets Firefox work on Vista with a few caveats outlined in the release notes. One of the important ones is that you can't set Firefox as the default browser. Another is that you can't install the browser to a directory name other than the default one the installer suggests. That's because Microsoft has added a "shim" in Vista to let it run in a backwards-compatible way, and the shim apparently recognizes applications by directory name. The shim will expire with version 126.96.36.199, so hopefully by then Mozilla developers will be ready by then to run in "native" Vista mode.
To track Mozilla's progress porting Firefox to Vista, or to find out where you can help, see bug 352420.
[Update: Since this article was written, considerable progress has been made to improve the browser for Vista. Read this updated article for more details.]