This seems to mean that a recording done on x86_64 MacOS can't be played back on aarch64 Linux, which is a shame for something that apparently is for javascript

This seems to mean that a recording done on x86_64 MacOS can't be played back on aarch64 Linux, which is a shame for something that apparently is for javascript debugging. Limitations like that are more understandable for rr, which is intended for use with native processes. It also looks like a maintenance burden which will increase the difficulty of supporting an OS for Firefox. Stuff like that causes Firefox to drop support for an OS, and it makes developers hesitant to add support for an OS. System calls can do all sorts of crazy stuff behind your back. Think of ioctl and fcntl. Just knowing what memory they modify is difficult. It's been lots of trouble for tools like strace and rr. Hopefully you won't encounter worse, like vfork and clone.