Are you one of the many suffering like this? A number of solutions have been provided here about the 203 nsExtensionManager.js error, all with varying results. But I think the most fail proof solution is slash and burn.
Update; So that no one misses the point. Deleting the extensions.rdf or replacing it may work depending on the corruption level of the config files in your profile. You are advised to try this before attempting the procedure below, which is meant to 'weed' out any and all corrupt config files to prevent the secondary recurrent bugs
THIS PROCESS TAKES 5-10 Minutes at the most.
(Firefox must be closed first before starting)
1. Go to your Tools>Add Ons
list down the names of all your extension on paper, (including the version)
2. Go to
Documents and Settings> User Name>Application Data> Mozilla>Firefox> Profiles,
find a copy of your latest back up bookmark file and save it somewhere easy to find like the desktop.( the one with today's date or the most recent one)
3. Go to Documents and Settings> User Name>Application Data
and delete the Mozilla folder. (leave no trace, delete also from recycle bin)
4. When you restart Firefox it is refreshed.. Select "Import Nothing" on the wizard that pops up.
5. You can then reinstall all the add-ons, WITHOUT any errors now.
(the note will help you remember which ones)
6. After, go onto the Bookmarks Organiser and import your bookmarks from the desktop.
Your Firefox will now work good as new.
I have posted a similar article on MozillaZine Forum. Hope this helps