Can you try to start Firefox in Safe Mode to see if Firefox works properly with no error? You can start Firefox in Safe Mode below:

• Windows/Mac: Go to Help > Restart with add-ons disabled
• Linux: Run firefox -safe-mode in the Terminal/Konsole

If Firefox opens up fine with no problems, it's probably one of your extensions that's causing the issue. You can re-enable your add-ons one by one until you find the one that causes the issue upon being re-enabled.

Nope. That didn't work. I only have AdBlock installed. No other add-ons. Nevertheless, safe mode didn't solve anything. I even installed and then reinstalled Firefox before asking the question, as well.

Please do the following:

Update to Windows 8.1 Update 1 if you haven't already. This probably isn't the cause of the problem, but this is good practice and housekeeping, plus it'll keep you more secure.

Then, create a new test profile and try to reproduce this issue there. Profile Manager - Create, remove or switch Firefox profiles

Can't install Windows 8.1. This is a work laptop and they have yet to approve the update to 8.1 (still on 8.0, unfortunately). However, it randomly started working when I restarted it without safe mode on. I'll use it for a while and see if the issue arises again.

If it happens again then try to close and restart Firefox and/or create a new window to see if it work is this new window.