Works fine here on Linux.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

• Don't make any changes on the Safe mode start window.

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

Interesting... I had already disabled Web Developer Toolbar, as it was the only add-on I've added or updated since Ctrl + Shift, Arrow was last working. Still, the keyboard works in safe mode, so something (firebug maybe?) must be intercepting that key combination & not passing it up to the browser.

Time for a process-of-elimination hunt I guess.

Thanks for the tip!

Sure enough... Firebug 1.6X.0b1 grabbed the keybinding, and not only when it has focus. Thanks for the tip!

You're welcome and thanks for posting the outcome of your 'hunt' for the culprit.

when firebug is causing this problem, a work-around is to specify the shortcuts for go back and forward in firebug (customize shortcuts -> go back & go forward) as something else.
as far as http://code.google.com/p/fbug/issues/detail?id=3721&q=gmail&colspec=ID%20Type%20Status%20Owner%20Test%20Summary clearing the shortcut in firebug will not work, but the issue is being fixed.

it helped on my system (win7, ff 3.6.13, tons of add-ons) when i just openend and closed firebug once.