Mozilla released Firefox 3.5 Preview, also known as Firefox 3.5b99, a couple days ago as a sneak peak at the upcoming release candidate. If you are running Beta 4, you can get it by checking for updates.
UPDATE 2009/06/11: WordPress 2.8 was released yesterday and this new version works well with Firefox 3.5 Preview. It also provides a lot of snazzy cool new features. Check it out if you can upgrade.