Note that using System Restore can cause your Firefox installation to get corrupted because not all files are restored (only files in a white-list), so be cautious with using System Restore.

• You may lose data in the Firefox profile folder like .js and .ini files.
• You may need to (re)install software that was affected.

Do a clean reinstall and delete the Firefox program folder before (re)installing a fresh copy of the current Firefox release.

Download a fresh Firefox copy and save the file to the desktop.

• Firefox 23.0: http://www.mozilla.org/en-US/firefox/all.html

Uninstall your current Firefox version, if possible, to cleanup the Windows registry and settings in security software.

• Do NOT remove personal data when you uninstall your current Firefox version, because all profile folders will be removed and you lose personal data like bookmarks and passwords from profiles of other Firefox versions.

Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.

• (32 bit Windows) "C:\Program Files\Mozilla Firefox\"
• (64 bit Windows) "C:\Program Files (x86)\Mozilla Firefox\"

• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.
• http://kb.mozillazine.org/Uninstalling_Firefox

Your bookmarks and other personal data are stored in the Firefox profile folder and won't be affected by an uninstall and (re)install, but make sure that "remove personal data" is NOT selected when you uninstall Firefox.

If you keep having problems then also create a new profile.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

Thanks for the reply - but I was having problems with ff version 23 and really do not want to reinstall it, besides I would lose all of my add ons if I uninstall version 20.

You wouldn't lose personal data like installed extensions as long as you wouldn't choose to remove personal data in case you would uninstall Firefox.

You should always be cautious with using System Restore because Windows doesn't show you which files are restored.
It is quite likely that some .ini. and .js files in the Firefox profile folder were restored to older versions.

What kind of problems did you have with Firefox 23?

Thanks much - uninstalled ff 20 and associated files and reinstalled ff 23. All working fine now - including previous problem with ff 23 now fixed.