the first troubleshooting step you can take is to run firefox in safemode & see if the problem is still occuring there. if not it's likely a faulty extension causing it - then you'd have to disable them, restart firefox and re-enable them one by one (restart might be necessary again) to find out which one is causing it - please report back afterwards, so that other users with the same issue can find a quick solution.

That helped me solve the problem, thanks for the tip. I was unaware Firefox had a Safe Mode - very useful! Thanks!

can you please tell us which of the addons caused the issue or how you did resolve it? thnx.

https://addons.mozilla.org/en-US/firefox/addon/whitespace-remover-for-goog/ - it was this add-on, I disabled it. The comments on the add-on page seem like its broken.

I have the same problem, but only while in gmail. Right column or area of browser page is clipped, showing white. I ran Firefox (14.0.1) in safe mode but problem still exists. I can clear this problem by hitting CTRL-MINUS to resize my page. I'll check google help and report back if solved.