So, no need to over-react. If Mozilla decides it's worth shipping, they'll port it to other plaforms.
* this is still open source. Fully open source.
* this is currently only available on MacOS
All that looks to have happened is the developers of this single feature that is in the nightly build are using Macs. It is wholly reasonable for them to only support one platform at this point. If it is a good feature they, or other devs attracted by this preview will port it.
"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."
In fact, their documentation makes a specific call-out to TTT (Time-Travel Debugging) on Windows; so, I suspect that this is the API that they would like to use but are unable to do so at present (presumably because many of the features are still lacking in the API, which Microsoft could - and most probably would - enable).
[0] - https://developer.mozilla.org/en-US/docs/Mozilla/Projects/We...
Looks to me like this is an experimental feature in Nightly. If it's highly system-dependent I can't blame them for testing the feedback as early as possible, before other systems are supported.
Question for Mozilla people here, if anyone sees it - is there a plan to bring this feature across all systems in time for regular Firefox release?