There is an override that you can enable.

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 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.

Apparently this fix would only work for a couple of weeks. It is not my job to contact the Xfinity/Comcast people, and have them sign something for Firefox. This is something that I should not be having to do. I can solve my issue, by doing the same thing that I did when Sprint refused to sign an agreement with Rogers Communications in Canada. I changed carriers. If I have to do Firefox' job here, then I will resolve my issue, by simply overwhelming them with my absence. I doubt they will care, but I'm not savvy enough on my computer to do all that. But I DO know how to use Internet Explorer.