Firefox with Greasemonkey: Gmail recently made a few tweaks to its message count API, leaving tools like the favicon message indictors by Eric Bogs and Peter Wooley, along with Gina's own Better Gmail 2 Firefox extension, broken. Now they're fixed.
Bogs and Wooley, whose clever user scripts cleverly show your unread message count in your Gmail or Google Apps mail's browser tab icon, both updated their code to adapt to Gmail's API changes. Gina, whose Better Gmail 2 extension utilises Bogs' and Wooley's scripts, has re-rolled those changes into her extension.
Those with Better Gmail 2 already installed should see update notices soon. If you're seeing only installation errors instead, hit the link for a direct (but non-updating) link. If you're rocking just Greasemonkey and one of these two scripts, head to one of their UserScripts.org pages and click "Install" to update your browser.
Better Gmail 2's Favicon Unread Count Fixed [Smarterware] Gmail Unread Message Count in Favicon for Greasemonkey Gmail Favicon Alerts 3 for Greasemonkey [UserScripts.org]