The simplest way to do this, what I believe memtest86 and friends do, is to write a fixed pattern over a region of memory and then read it back later and see if it changed; then you write patterns that require flipping the bits that you wrote before, and so on.

Things like [1] will also tell you that something corrupted your memory, and if you see a nontrivial (e.g. lots of bits high and low) magic number that has only a single bit wrong, it's probably not a random overwrite - see the examples in [2].

There's also a fun prior example of experiments in this at [3], when someone camped on single-bit differences of a bunch of popular domains and examined how often people hit them.

edit: Finally, digging through the Mozilla source, I would imagine [4] is what they're using as a tester when it crashes.

[1] - https://github.com/mozilla-firefox/firefox/commit/917c4a6bfa...

[2] - https://bugzilla.mozilla.org/show_bug.cgi?id=1762568

[3] - https://media.defcon.org/DEF%20CON%2019/DEF%20CON%2019%20pre...

[4] - https://github.com/mozilla-firefox/firefox/blob/main/toolkit...

[4] looks like it's only a runner for the actual testing, which is a separate crate: https://github.com/mozilla/memtest

(see: https://github.com/mozilla-firefox/firefox/blob/main/toolkit..., which points to a specific commit in that repo - turns out to be tip of main)

That would tell you if there's a bitflip in your test, but not if there's a bitflip in normal program code causing a crash, no? IIUC GP's questions was how do they actually tell after a crash that that crash was caused by a bitflip.