This should help you, at least far as that old extension not being signed. But this isn't a permanent fix. It won't work in the next Firefox 44 version. Cool Previews has been discontinued by the Cool Iris crew - so it's dead. https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox#w_what-can-i-do-if-firefox-disables-an-installed-unsigned-add-on '''Override add-on signing (advanced users):''' You can override this setting by changing the '''xpinstall.signatures.required''' preference to '''''false''''' in the [https://support.mozilla.org/en-US/kb/about-config-editor-firefox Firefox Configuration Editor] (about:config typed in the URL bar). Support is not available for any changes made with the Configuration Editor so please do this at your own risk. Note: That pref will be gone in Firefox 44 (Release slated for Jan 26. 2016) so user's of currently un-signed extensions should contact the developers of those extensions and prod them into getting their act together & get their extensions signed by Mozilla. Otherwise the users of those extensions will be SOL come the release of Firefox 44. ''Mozilla has already pushed back the version in which this new security feature was to be enabled from Firefox 40 to 44. I doubt if there will be any further reprieve.''

many thanks for all your help too bad about cool previews being dead I used it many time each day :(