One keystroke that I use a lot when typing on a computer is Control+Shift+[an arrow key] but I found myself in the awkward position of it not working in Firefox anymore. The nuisance level was enough to set me investigating in the name of resolving the problem. Using the following command to start Firefox saw the keystroke being returned to me so I need to find which plug-in, extension or add-on was the cause of the matter.
Then, it was a matter of disabling one extension at a time and restarting Firefox each time to see when the keystroke functionality was returned to me. The culprit turned out to be Firebug 1.6 and there’s a discussion on their bug forum about the issue. Even the good folk in the Firebug project noted how many folk were experiencing the inconvenience based on a quick Google search. However, that didn’t turn up the answer for me so I had to do some digging of my own and I hope that it has saved you some time. Of course, Firebug comes without cost so we cannot grumble too much but I’ll be keeping it disabled as much as possible until a new version makes its appearance.
Update 2011-01-15: This now seems to be fixed in Firebug 1.6.1