Do a clean reinstall and delete the Firefox program folder before reinstalling a fresh copy of Firefox.

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

• http://kb.mozillazine.org/Installation_directory

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

• Firefox 18.0.x: 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 will also lose your 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.

• 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 profile 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.

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

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

I followed the steps you have suggested, when I started FF 18.0.1 I still received the error "Could not load XPCOM". I reran this process to delete and remove FF 18, then ran Malwarebytes, no problems found, and cleaned the registry, then reinstalled FF 18. again the error "Could not load XPCOM" appeared again. I have reverted back to FF 17. Any other ideas to resolve this?

Thanks

You did delete the Firefox program folder, because such issues are usually caused by a failed update to some files?

Yes I deleted the Firefox program directory as per instructions. I think I will try a custom install and select a different directory for the program installed location.

I just installed Firefox 18.0.1 in a different directory and Firefox now starts error free. Thanks for your support cor-el your input lead me to the solution,

You're welcome