This may be because the add-on is not longer supported by the latest version of Firefox. You can try to see if a newer version of the add-on is available and can be downloaded. If not, contact the developer so they can inform you of alternatives or when the add-on will work again.

Thanks!

The latest version of Firefox being 40.0 www.mozilla.org/firefox/all as 41.0 is currently on the Beta channel with current build being the first beta 41.0b1.

Firefox 40: Firefox warns about signatures but doesn't enforce them. Firefox 41: Firefox will have a preference that allows signature enforcement to be disabled (xpinstall.signatures.required in about:config). Firefox 42: Release and Beta versions of Firefox 42.0 will not allow unsigned extensions to be installed, with no override.

By testing the Beta builds (of 41.0) you are seeing the changes sooner.