FYI, the really cool Web Developer Toolbar extension for Firefox does not appear to be 100% compatible with the new Version of Firefox. For me, every url entered in the address bar triggers a separate tab visiting the w3 html validator. Pretty annoying, to say the least. This had me freaking out looking for all sorts of viruses and spyware without turning anything up.
After I uninstalled the extension, the problem is gone. I took a look at my traffic with ethereal, and there is no strange traffic, so I think that got the problem.
I notice that there is a little asterisk next to the 1.5 entry on the FireFox extensions page for the toolbar, so I guess that should give us a heads up as well.