This can be caused if one of the files that stores details of installed extensions is corrupt.

1. Open your profile folder, for details of how to find it see https://support.mozilla.com/kb/Profiles
2. Close Firefox
3. Delete the following files if they exist, extensions.cache, extensions.rdf, extensions.ini and extensions.sqlite
4. Re-start Firefox, it will then rebuild the list on installed extensions

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Sorry about the first reply it had errors in it and was duplicated by this message. Thank you for reply. I will let you know if this has solved the issue.

The only negative result I have seen as all of the add-ons that use the "Add-on Bar" have been rearranged. Which I could figure out how to get them back the way the were. :-).

George

Sorry - I misunderstood your unmovable add-on icons problem. I have the same problem with Greasemonkey & NoSquint - can't figure out why they can't be moved rearranged like add-on icons that I manually add there.