The fix is to uninstall Firefox completely [First, back up your bookmarks]. The error is caused by an add-on. If you get the Div Null you're unlikely able to open your add-on manager to remove any. When you uninstall the program, make sure to check to delete all your bookmarks and personal data. Then reinstall Firefox and the problem is fixed :)

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

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

Start in Safe mode with all add-ons & extensions disabled. Try youtube, if ok check for a Youtube download helper or mp3 helper. I removed this and Youtube then worked fine.

I fixed this by disabling the YouTube MP3 Downloadhelper plugin.

I had the same issue, removed that addon and problem solved. No need to uninstall

I got rid of this problem by disabling the addon:

Easy YouTube MP3 Downloader 1.7 by Kaaj

Link to the addon page: http://tinyurl.com/77k6j7u

There are several GOOD YouTube downloaders available. Make sure this is the one you have before you disable it.

running: CometBird 7.0.1 alternative to Firefox (but is uses the same engine--in fact everything is the same as Firefox, but with a different name, and built-in support for the BitComet client without using an addon after they stopped development on the addon itself--I guess they did this to start pimping their own web browser. Whatever... it does seem to eat up less RAM, though.)