That script error has been associated with one of AVG's Firefox add-ons. If you reinstall Firefox 5, you probably can disable that add-on and run normally.

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Regarding earlier versions, due to security vulnerabilities, rolling back to Firefox 4.0.1 is not recommended. Firefox 3.6.18 would be the safer choice.

Here's the process to roll back:

First, I recommend backing up your Firefox settings in case something goes wrong. See Backing up your information. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, download and save Firefox 3.6 to your desktop for future installation. http://www.mozilla.com/firefox/all-older

Close Firefox 5.

You could install Firefox 3.6 over it (many have reported success) or you could uninstall Firefox first. If you uninstall, do not remove your personal data and settings, just the program.

Unless you have installed an incompatible add-on, Firefox 3.6 should pick up where you left off. If there are serious issues, please post back with details.

Thank you for your response and help. How do I know , which add-on on AVG's is causing the problem?....should I disable the entire AVG during the downloading of version 5?

The main AVG program is fine, so no need to disable that.

I recommend backing up your Firefox settings in case something goes wrong with the next step. See Backing up your information. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

After installing Firefox 5, hold down the Shift key when starting Firefox. This will open Firefox in Safe Mode. You can check the box to disable add-ons so you can start Firefox without the AVG add-on running. (To avoid losing custom settings, don't check the reset or delete boxes in that dialog.)

Once you have Firefox started in a stable condition, you can re-enable your most critical add-ons here:

orange Firefox button or Tools menu > Add-ons

Make sure to review both the Plugins and Extensions.

Does it work?

Thank you again....I will try your instructions tomorrow....because here it is 2:20 AM....you see I'm in Florida...I'm falling sleep....tomorrow I'll let you know the result....have a good night!

Dear jscher2000: I have fallowed your instruction to download firefox five, then pres Shift, before open it and star in safe mode....then I disable add on. It seem to be workimg just fine....I can't thank you enough for your help. New question: should I always has to press the shift key, before open firefox, from now on or just simple star it?. Please forgive my ignorance in this regard. Thanks a million. Juan

Hi Juan, you only need to hold Shift to use Safe Mode, which is just for testing. If you disabled your add-ons, then you can just start Firefox normally.

Hi srcher2000....I understand....thank you a lot...."muchas gracias"