Does it help if you make the Add-ons bar visible?

View > Toolbars > Add-on Bar

I indicated that I have this problem too, or at least a related one, but I wanted to add some details. In my case, using gmail, the resize button actually isn't even visible. But when I try to put the mouse on the line-scroll arrow at bottom right, the cursor changes to the resize cursor, and I can resize from there (with the mouse a good bit above where the resize button should be). So I think it's the same problem reported by this user. As indicated, it's impossible to use the line-scroll button in this mode. I don't seem to have the problem on other web pages, or at least not that I've noticed. I just obtained Firefox 4, so I think I've got an up-to-date copy, and I'm running on Windows XP, SP2. And other than this, I *really* like FF4--that tab organization feature is absolutely stunning in its usefulness. Oh, and, in case it matters, I'm also running the gmail under the "pinned tab" option.