hello, this sounds like the files in your profile folder that store the information about your extensions might have become corrupted. please go to firefox > help > troubleshooting information > profile - show folder... then windows explorer should open up your profile folder. in there look for the files named extensions.ini, extensions.sqlite, extensions.sqlite-journal (and in some cases also extensions.txt, extensions.rdf). delete or rename those files and quit firefox, they will be regenerated the next time you launch firefox. at this occasion also check that the contents of the profile folder are not write-protected (by right-clicking it > properties).

also see Corrupt extension files

Yay!

Thanks madperson for nudging me in the right direction.

I tried to delete the files and could not - windows prevented me, saying that I needed permission from myself to delete them. It turns out that my virus software was causing this.

I disabled the Access Protection on the McAfee Virus Scan Console and was able to install the addons. I did not need to delete the extension files.

Thanks!