Mozilla launched a project called the recent update of Firefox Nightly version of Firefox Replay experimental tool that allows Firefox content recording process their behavior, reproduction and later fall back to an earlier state. Replay process preserves all the same behavior JS, DOM structure, graphics updates, as well as other acts. Browser JS debugger can be used to check and control the replay. This feature is currently only supports macOS.
MDN (Mozilla Developer Network, Mozilla developer site) page document in this function is called Web Replay, due in beta, it is disabled by default, you can use the manual devtools.recordreplay.enabled preference is on. By "Tools" -> "Web Developer" menu, see "Recording / Playback" tab, enable the new debugger interface, you can access. The document also mentioned that some of the content is still in pre-alpha stage.
Firefox Replay development team members jasonlaster11 network also posted responded to everyone's questions. He said that the feature is currently in Phase users, once they are included in the official road map, it will definitely give priority to cross-platform support. Official website firefox-replay.com also is under construction, it has not been officially released. In addition, for the user's concerns, he must also ensure that Replay is open source.
Through this new class of tools, Firefox, or for developers to bring better web debug experience. For more information about Firefox Replay, you can see the MDN documentation:
https://developer.mozilla.org/en-US/docs/Mozilla/Projects/WebReplay