They were told to not anymore host the extension on AMO, but it's still signed by Mozilla (and I doubt they would stop signing future versions), so you can still download and install the extension from elsewhere.

Also, Mozilla themselves provide a "fork", i.e. builds compiled with the corresponding flag, that allow unsigned extensions to be installed, after you set "xpinstall.signatures.required" in about:config to false. These builds are Firefox Developer Edition and Firefox Nightly, as well as Unbranded Builds [0] for Beta and Stable.

I believe most Linux distributions also compile with this flag, as disallowing unsigned extensions mostly helped against malware on Windows.

[0]: https://wiki.mozilla.org/Add-ons/Extension_Signing#Unbranded...