I finally got around to installing Firefox 3 on my laptop just now, and have spent the last several minutes trying to figure out why my userchrome wasn’t working like it did in 2.x. I’m used to having all buttons, menus, and the address bar on a single line at the top of the window, so that I have more vertical space to display content. See the screenshot below.

Firefox with modified userchrome
That means I need to cut out unnecessary menu items like the Help, View, Edit, History, and Bookmarks. (I use the collapsible All-in-One Sidebar add-on for all that functionality.) Anyway, after installing FF3, the Bookmarks menu came back. I checked my userchrome.css file and it was the same as before. Then I had to google for a good 10 minutes before I found a user comment that mentioned FF3 had changed the CSS name for the Bookmarks menu from #bookmarks-menu to #bookmarksMenu. Ugh, why?