Your resolution sucks at life, get a bigger monitor/better video card.
I don't have money to blow on a video card I'll never use for anything this one can't do.
My 17-inch monitor is quite nice, thank you.
If 1280x1024 sucks at life I'd hate to see what you're using for a monitor.
[...] Even if there is plenty of space, now matter how big I make the window, the scrollbar is still there.
Yeah, the picture doesn't do a great job of showing this, but that's the problem.
It's the fault of the theme or the browser parsing the theme, obviously.
Changing the theme did nothing.
I've never experienced this personally, so I'd recommend reinstalling Firefox?
Tried reinstalling Firefox (well, upgrading, but the problem is in both versions). If you're a Linux user, what window manger do you use? I'm using GNOME.