Iuru pointed me to a plugin on userstyles.org that fixes the issue Mac Favicon was designed to resolve in a far more maintainable and cleaner way than Mac Favicon ever could. Therefore I've decided to deprecate Mac Favicon and Mac Favicon XL themes for Firefox and instead encourage users to install the Stylish addon, then install the Mac bookmark toolbar favicons style from userstyles.org.
If you prefer Mac Favicon XL, see the instructions that follow.
UPDATE: Mac Favicon and Mac Favicon XL have been deprecated and rereleased. See http://drupal.geek.nz/blog/mac-favicon-deprecated-and-released
With Firefox 3's release now imminent, a number of people have contacted me about whether there will be a version of my Mac Favicon theme for FF3.
While the short answer is "yes", the long answer is more complicated; I won't be able to take this on anytime soon -- probably not till August. A couple of folk have contacted me about doing it themselves. I would be really happy if someone were to do it. I would happily test it, release it on addons.mozilla.org under Mac Favicon, give you access to do that or link to your release elsewhere.
I just got this email from Mozilla about the soon release of Firefox 3 release candidate, asking addon developers to get their extensions updated and released for FF3 RCs. If you have an extension released on addons.mozilla.org you would have received this email too. I have two variations of FF2's default Mac theme that enable favicons in the bookmarks toolbar: Mac Favicon and Mac Favicon XL.