Ok, so it seems to have stopped? I pressed and held down the shift key and it seems to have resolved the problem. That's strange, I don't think my shift key is stuck as I was able to type just fine, play League of Legends, and more. I guess if anyone has this problem and comes across this, give the shift key a firm hold?

It is possible that some of the status keys have the wrong state and Firefox sees them as still pressed.

Try to press all the status keys (Shift, Ctrl, Alt) a few times to see if that helps to recover from this. If it doesn't help then close and restart Firefox or reboot the computer.

Closing Firefox and rebooting the computer did not do the trick, but pressing the Shift status key did fix it! Thanks for the quick response!

Weird that this survived a reboot. So in your case you would have to power down the computer,

Did this affect other software as well?

That's whats very strange about this. I could be mistaken and perhaps the computer was sleeping and not shut down, but I do think it likely was shut down. I can be quite oblivious though. But I even uninstalled, and removed the remnants of the local app directory after uninstalling, and then reinstalled. So the only thing I think it could have imported was from my synced account.

And no, I noticed no other problems in any application, such as Chrome which I had to use for obvious reasons, and League of Legends. Surely if the Shift key was activated it would have interfered noticeably in both of those as well.