Mozilla plans to switch the legacy add-ons system of the Firefox browser off in the Firefox 57 release which will be out in November.
The organization focuses on WebExtensions instead, and adds new APIs and improvements to Firefox to extend the capabilities of WebExtension add-ons.
Mozilla wants feature parity with Chrome when it comes to WebExtensions, and extend the capabilities of the Firefox browser in regards to WebExtensions beyond what Google Chrome offers.
This means effectively that any Chrome extension can be ported with relatively easy to Firefox, and that Firefox add-ons will become available that offer functionality that Chrome extensions cannot replicate.
Mozilla makes Tab Hiding API a priority
Mozilla approved the Tab Hiding API yesterday and made it a priority 1 project. This API extends WebExtensions support in Firefox beyond what Chrome supports. As you may know, Chrome provides little to no options for add-on developers to manipulate the browser UI. Even core Chrome does not support modifications to the tab bar, for instance to change the disastrous "no scrolling on tab bar" rule of the browser.
The best Chrome extensions can do is to remove tabs from the browser's tab bar to display them in list form. Extensions like TabSense, PanicButton or Simple Window Saver improve the tab handling for instance.
Firefox's upcoming Tab Hiding API paves the way for Tab Groups like add-ons. Firefox has its fair share of Tab Group legacy extensions; Simplified Tab Groups or the excellent Tab Groups by QuickSaver are two of the most popular add-ons that provide the functionality.
The author of Simplified Tab Groups mentioned already that he would port his extension once the APIs become available.
Basically, what the API provides is functions to show and hide tabs in the Firefox tab bar. This is the main use case for tab groups functionality, a feature that allows Firefox users to create groups of tabs and switch between them.
Since you work only with a subset of tabs at a time, it means that you have a better overview of the open tabs of that group usually.
Mozilla has all hands on deck for the important Firefox 57 release. It seems likely that the pressure will drop once Firefox 57 is out, and the organization will continue to improve and add APIs to the browser.
It is clear that these APIs will never reach feature parity with the legacy add-on systems though.
Raymond Hill, the creator of uBlock Origin published an interesting post recently in which he confirmed that Firefox will still be superior to Chrome when it comes to content blocking when Firefox 57 gets released.