A major change happened to Firefox in recent months is: Starting from Firefox 57 (released on 2017-11-14), legacy add-ons will no longer be supported. Legacy means those add-ons are not developed using the WebExtensions technology.

To be more precise, here’s Mozilla’s official declaration:

Starting from Firefox 53, no new legacy add-ons will be accepted on
addons.mozilla.org (AMO) for desktop Firefox and Firefox for Android.

Starting from Firefox 57, only extensions developed using WebExtensions APIs
will be supported on Desktop Firefox and Firefox for Android.

A direct consequence is the loss of many useful add-ons. At least we have to wait until add-on authors migrate their projects to WebExtensions. On the one hand, some of them were able to do it quickly, but the result looks nothing like its predecessor. On the other hand, some excellent projects have given a clear end-of-life statement.

So what are your options? Downgrade.

Fortunately, Mozilla keeps all historical versions of Firefox on their CDN and FTP:

Any version before Firefox 53 should work well with legacy add-ons.