UPDATE! Fixed. It was caused by an add-on, "Tab Information" or somesuch that I had disabled. So I guess this is a separate FF4 bug where it re-enables add-ons randomly (had it do it with tab tree view, too). Actually, now that I think of it, that's a much worse bug. :-/
Firefox 4 is out! Big congratulations and much gratitude for their hard work in what is looking like a pretty great update. But the change to the UI regarding tabs is freaking me out.
The (live!) thumbnail is really neat, but I'm not sure if it enhances usability. Worse, the contrast for the currently selected tab isn't great enough to tell at a glance which tab is currently active. I think this is a bug as it should probably be seamlessly drawing into the navigation toolbar below it. Also, I think they have a drawing bug with the single-pixel horizontal line between the tabs and the navigation toolbar below it:
Ok. Point-zero releases inevitably have minor issues so I can ignore it. If these aren't drawing bugs and are instead what they want the UI to look like, that's a pretty bad mar on an otherwise great piece of software. I guess we'll see how they handle the flood of bug reports.