By official site you mean its homepage https://www.readability.com/addons as it appears to not be hosted on https://addons.mozilla.org/firefox/ where it would be signed.

Current Firefox releases require extensions to be signed to prevent abuse by malware. As of Firefox 43 and newer unsigned extensions will be disabled by default. Firefox 38 shouldn't be affected normally.

You can set xpinstall.signatures.required to false in about:config to disable this security feature in Firefox 44.

With the Release and Beta builds of Firefox 46 and later you can no longer override this.

Is this the extension that you are talking about? https://addons.mozilla.org/en-US/mobile/addon/readability-47136/ Doesn't appear when doing a Search of AMO using the complete name. It flashes in the drop-down when the search term is lacking the last letter - IOW, when readabilit is typed, note the 'y' is missing. Some sort of fluke that I even found that old extension at all.

It's an ancient extension for Mobile Firefox aka Fennec in those days and hasn't been updated since 2010 (11.0a1 last compatible) - not signed. The AMO website needs to be cleaned up, but with the change to Market Place 'cleaning' may not be needed.