Just heard about Firefox "Safe Mode", so I tried that.

Things worked fine in Safe Mode, and I recalled that the add-on Firebug just installed an update. I disabled it and things are all good now.

So I disabled Firebug and this resolved the issue too. I sure hope Firebug releases a fix for this soon, as it's a tool I use quite a bit.

Thanks for the advice tmdeppner!

I hope they fix it soon, too.

If you want to follow the developers' discussion, there's a thread about it in the Firebug issues forum: http://code.google.com/p/fbug/issues/detail?id=3721

OK - it's firebug but you don't need to throw out the baby with the bathwater and uninstall it.

The problem is key bindings. Basically when you do Control-Shift-Left (and Control-Shift-Right) firebug has those bound to "forward" and "backward" commands. You can fix those bindings and then they won't bug you again.

Click:

Tools -> Firebug -> Customize Shortcuts -> click on the "Go Forward" link and press Control-Shift-ALT-Right Arrow, then click on the "Go Back" link and press Control-Shift-ALT-Left Arrow.

Then click OK.

If you don't have firebug installed then it may be another plugin that is causing the issue...

Nice to have a work-around until it's fixed. Thanks, uptoeleven.