The following guide lists all options that you have to continue running so-called legacy add-ons when Firefox 57 is released in November.
Mozilla plans to disable the legacy add-on system in Firefox 57 Stable. This means that any extension for Firefox that is not a WebExtension, the new format that Mozilla focuses on, will stop working in Firefox 57.
Many add-ons that are available for Firefox right now will stop working. New extensions are created, but it is fair to say that WebExtensions will never be as powerful as Firefox's legacy add-on system.
We made a rough analysis of the situation in July; it is improving, but there are still plenty of legacy add-ons out there that are not available as WebExtensions yet. Some add-on authors may be working on a port, some may wait for APIs to become available, and some have announced already that they won't port their extensions (usually because it is not possible, and won't be possible).
Firefox users who rely on specific legacy add-ons have a couple of options to retain their functionality. The following options are available:
Switch to Firefox ESR (until June 26, 2018)
Firefox ESR, Extended Support Release, will support legacy add-ons until June 26, 2018. This special version of Firefox is maintained specifically for organizations that require more stability and less change when it comes to software.
Firefox ESR gets all the security updates that Firefox Stable gets, but none of the functionality changes. The current version of Firefox ESR, 52.x, is based on Firefox 52.0. This means that it won't incorporate any changes made in Firefox 53, 57 or even 60.
It is no longer supported after the release of Firefox ESR 52.8.0 which is released next to Firefox 60.
Switch to Firefox Nightly (unknown)
While Firefox Release and Beta versions won't support legacy extensions anymore when Firefox 57 hits the channel, the same cannot be said for Firefox Nightly.
Mozilla plans to ship Firefox Nightly with a preference to enable support for legacy add-ons.
This sounds good at first glance, but changes in the browser's core will render add-ons non-functional in Firefox 57.
Jorge Villalobos, product manager for addons.mozilla.org, confirms as much on the official Mozilla blog:
Starting with 57, many things will break that will also break add-ons. Most add-ons that aren’t WebExtensions will be broken regardless.
Compatibility issues will grow over time when further changes are made to the Firefox web browser. While some add-ons will continue to work at least for a while, it is unclear for how long, and for how long Mozilla plans to keep the preference in Nightly to keep on using legacy add-ons.
Block further updates (unlimited, but insecure)
Another option that Firefox users have is to block any updates to the browser so that Firefox won't get updated to version 57.
While that removes the time limit for using any legacy add-on that still works in Firefox 56, it means as well that Firefox won't get updated anymore with security updates. This makes it a non-option for most use cases.
Firefox users who want to disable automatic updates need to load about:preferences#advanced, switch to the advanced tab there, and select "never check for updates".
Note that Mozilla changes the preferences layout.
Pale Moon (not all add-ons are compatible)
Pale Moon, which shares a lot of code with Firefox, may be an option of the add-ons that you want to use work in the browser.
Not all Firefox add-ons are compatible with Pale Moon on the other hand, so you best verify this before you migrate to the browser.