If you are a Firefox fan than you must have experienced periodical out-dating and temporary unavailability of your favorite extension every time you update your browser. Same thing is going on with the upcoming release of Firefox which is Firefox 57. It will release from November 2017 but if you are on beta channel you can upgrade to it now, from October 2017. If you have upgraded you will notice many extensions are not compatible and since disabled by-default and there is no option available to enable them. These extensions are listed as "Legacy Extensions" which used to be listed as incompatible extensions in previous versions.
From Firefox Official Blog:
In the past, extensions often stopped working each time a new version of Firefox was released, because developers had to update them every six weeks to keep them compatible. Since extensions could also modify Firefox internal code directly, it was possible for bad actors to include malicious code in an innocent-looking extension. To address these issues, and as part of broader efforts to modernize Firefox as a whole, we’ve transitioned to a new framework for developing Firefox extensions. Outside of rare instances, extensions created with the new standard won’t break in new Firefox releases. You can still personalize Firefox with extensions the same way you do now, except they won’t break in new Firefox releases.
I found couple of essential extensions in this list like Adblock Plus and Browsec VPN. I did some lookup and the alternatives you can use are AdBlock for Firefox (https://addons.mozilla.org/en-US/firefox/addon/adblock-for-firefox/?src=ss) and Best Proxy Switcher (https://addons.mozilla.org/en-US/firefox/addon/best-proxy-switcher/?src=search).
To find alternatives click on "Find a Replacement" button next to the extension and you will be redirected to Firefox add-on pages listing alternative extensions listed under that category.