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Mozilla is constantly putting its efforts into improving Firefox’s devtools. One such effort is Firefox Replay, an experimental tool that allows Firefox content processes to record their behavior so that it can be replayed and rewound later.

The main highlight of Firefox Replay is the “code timeline” that enables you to scan through every code execution at a glance. Along with execution points, the timeline also shows exceptions, events, and network requests in real-time. It also allows you to save your recordings and pick up where you left afterward.

How Firefox Replay works

The record and replay behavior is achieved by “controlling the non-determinism in the browser.” Initially, it records non-deterministic behaviors (intra-thread and inter-thread) and then replays it later to “force the browser to behave deterministically.”

Firefox Replay includes IPC integration to enable communication between a recording or replaying process and the chrome process. Its rewind infrastructure allows a replaying process to restore a previous state. Its debugger integration enables the JS debugger to read the required information from a replaying process and control the process’s execution.


Firefox Replay is not officially released yet, however, Mac users can give it try by downloading the nightly builds. Since it is still experimental, Firefox Replay is disabled by default. You can turn it on with the ‘devtools.recordreplay.enabled’ preference.

Read also: Firefox Nightly browser: Debugging your app is now fun with Mozilla’s new ‘time travel’ feature

The team is working on support for other platforms as well. “Windows port work is underway but is not yet working.  The difficulties are in figuring out the set of system library APIs to intercept, in getting the memory management and dirty memory parts of the rewind infrastructure to work, and in handling the different graphics and IPC pathways on different platforms,” the official doc reads.

In a discussion on Hacker News, many users were excited to try out this tool. A user commented, “This might be enough to get me to use Firefox to develop with. This could be huge for its market share, a big part of the reason chrome was able to become so popular was because of how good its devtools were (compared to the competition at the time).

Firefox definitely managed to catch up but not before lots of devs switched to chrome and stopped checking for compatibility with Firefox.

This will be an absolute game-changer for web development. I am currently working on a really simplified version of this but as a chrome extension. We deal with a lot of real-time data and have been facing some timing issues (network and user input) which is really hard to reproduce,” a user added.

Check out Mozilla’s official docs to know more in detail.

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