Ifound this link, and have been working through it No joy yet, but may help others.http://kb.mozillazine.org/Unable_to_ins ... _-_Firefox
I have also found this reply to the same question
There is an open bug report at Mozilla regarding this error message and so far there appears to be no definitive solution.
For some people it has been resolved by following the instructions for corrupt extension files at Unable to install add-ons
Others have had to create a new profile (Firefox stores user data and extensions in profiles)
Creating a new Firefox profile on Windows - MozillaZine Knowledge Base
If the new profile solves your problem you can then transfer some data such as bookmarks, passwords, cookies, personal dictionary, security certificates etc to the new profile. Do not copy the extensions, instead it is better to re-install them.
Transferring data to a new profile - MozillaZine Knowledge Base
For some even this does not work.
You could also try downloading the extension (right-click on the install option and choose Save Link As...), then use File > Open File... and point it to the file you just downloaded.
I have tried to modify profile folder as suggested with no luck, I deleted the mozila folder, then removed FF using spritz and did a clean install. Back where I started from. If I right click on the download and "save link as" recieve a download error.
Not life or death that I have the 2-3 add -ons I want, all else works fine. I will keep playing with it and hope a update comes along.