Perhaps the whole-world-but-me already knows this... but I'd really never known I could refresh (Ctrl-R) directly at Firefox's View Source window!

Write some code > View with browser > View source and see generated HTML
Make code changes > Refresh view-source window and see updated HTML

Now that's quite a dandy feature! Only found out just now when I'd accidentally refreshed the "wrong window" (so if you'd already known it, its only because you're clumsier than me. Tsk!)