Go the your home folder. Find the hidden folder .mozilla. You need to have View|
Show hidden Files checked in Nautilus to do this. Then look for Firefox and open
that folder. Then find your user folder. It will have a bunch of alphanumerics
followed by .default. Mine looks like this: w9x9og1.default. Open it and look
for the folder Extensions. Open it and delete all of the extensions. You may
have more than one user folder if you have installed and de-installed Firefox.
You can delete them all.

Then try re-installing the extensions that you want. There is no need to install
Firefox. Just deleting the extensions should work. If this fails delete all of
your Firefox folder in the .mozilla folder. It will give you a fresh start.