It's possible that you are having a problem with some Firefox add-on that is hindering your Firefox's normal behavior. Have you tried disabling all add-ons (just to check), to see if Firefox goes back to normal?

Whenever you have a problem with Firefox, whatever it is, you should make sure it's not caused by one (or more than one) of your installed add-ons, be it an extension, a theme or a plugin. To do that easily and cleanly, run Firefox in safe mode (don't forget to select Disable all add-ons when you start safe mode). If the problem disappears, you know it's from an add-on. Disable them all in normal mode, and enable them one at a time until you find the source of the problem. See this article for information about troubleshooting extensions and themes and this one for plugins.

If you need support for one of your add-ons, you'll have to contact its author.

If the problem does not disappear when all add-ons are disabled, please tell me, so we can work from there. Please have no fear of following my instructions to the line, as all can be easily undone.

I seemed to have fixed my problem by running FF in safe mode, disabling all add-ons. The error message appeared again, but I closed the error dialog box and started a new FF session where FF says it's embarrassing--could not restore previous session. That seemed to fix the problem. I could restart FF the next time with all add-ons loaded and the problem did not reappear. Running FF 3.6.12 on Windows XP.

Hi Morbus,

I was getting the same Failure Code and followed your instructions. It did take care of that problem, but now I can't view You Tube videos, Hulu, and a bunch of other sites that I use regularly. I tried to re download the Flash player for You Tube but they still won't play. How can I start adding add ons back in to Firefox?