Well, about an hour ago I checked in some code into the Growl subversion repository for an extension that will get Growl working with Firefox. There is still a minor issue with it, and sadly the issue prevents it from working, but I think that can be solved sooner rather than later.
As of now, the only thing it does is enable the download complete notification to work. You might say “but we’ve already got that!“, but I assure you that my approach is much better. First of all, it doesn’t require that you have growlnotify installed anywhere. It uses the same code that I’m using for Bug 362685, with a few changes for it to work on the 1.8.0 and 1.8 branches (Firefox 1.5 and 2.0 respectively).
I’m starting off small, but it can get much more advanced as things go on. This will, of course, be more useful for extension authors (as consumers) as opposed to browser functionality (the only thing nsIAlertsService is used for is the download complete notification). I can add more notifications down the line though, and I also plan to support at least Thunderbird, and maybe even Sunbird too! I’ll try to keep anyone interested in this updated here.