Defender (or other AV) can slow down a lot of things, but in terms of the exact way that Firefox ran into it, the other apps would be anything with a JIT. Well,

Defender (or other AV) can slow down a lot of things, but in terms of the exact way that Firefox ran into it, the other apps would be anything with a JIT. Well, a JIT that uses memory protection as a security measure, though that's very common. (After generating executable code, the JIT marks the pages as executable but non-writable, so an attacker can't change the code after it starts running.) Although the V8 JIT stopped using this, at least in some configurations (?), for the stated reason that it's not perfect—another thread could sneak in and modify the executable code in between when it was generated and when it is protected in preparation for execution. They're instead planning to rely on memory protection keys, which should be faster and more robust, but are only available on some hardware. JITs can show up in unexpected places. Regular expression engines will sometimes have a JIT. So... I don't know?