When Mozilla released Firefox 57 back in November 2017, support for the new WebExtensions system for browser extensions was limited.

Lack of support for certain APIs was a big problem as it meant that some extension developers could not port their extensions to the new extension system at all, had to omit features to do so, or wait for APIs to become available to produce a port.

The issue would not have been that problematic if Mozilla would not have made the decision to remove the old add-on system in Firefox 57.

New WebExtension APIs were introduced in newer versions of Firefox and others were improved to include new or missing functionality.

While it is fair to say that the WebExtensions system willl never support the same capabilities that Firefox's classic add-on system supported, it is clear that Mozilla is making progress and that the new extension system has become more powerful since the release of Firefox 57.



Mozilla revealed a list of WebExtension APIs that it plans to include in coming versions of the Firefox browser.

Starting with the release of Firefox 62, the following WebExtension APIs will be introduced in Firefox:

API
target release
userScripts
63
topSites
62
desktopCapture (TBD)
63
declarativeContent
63
Session management
63 (TBD)
Toolbars
63 (TBD)
Overlays
64 (TBD)

There is also discussion going on about introducing a color filter API. Mozilla did not reveal any other information about the upcoming APIs but the API names hint at what those could provide.

Note that the following is my best guess based on the name and some information provided by Bugzilla and Mozilla Wiki pages.

• userScripts -- management of userScripts and support for running userscripts in sandboxes and make userscripts more reliable See bug 1437098 and Wiki.
  
• topSites -- extend the functionality of the topSites WebExtensions API. See bug 1446915
  
• desktopCapture -- implementation of Chrome's chrome.desktopCapture API. See bug 1303919
  
• declarativeContent -- to match Chrome's declarative Content API. See bug 1435864.
  
• Session Management -- give extensions control over sessions. See bug 833791
  
• Toolbars -- unclear, maybe control over existing toolbars and options to move icons and such?
  
• Overlays -- secure overlay API to inject without "spamming the website DOM". See bug 1340930.
  

Some APIs add much requested functionality; session management is probably the number one here as extensions cannot control the functionality directly in Firefox currently. Extensions like Session Boss help themselves by using their own load and save functions.

Source