Yes, I have read the bug report. It mentions that Firefox writes wayyyyy too much in the temp folder. It also mentions that the team should fix this behaviour independently of the fact that some of those calls are more costly than they should be because of the bug in Defender:
> With a standard Firefox configuration, the amount of calls to VirtualProtect is currently very high, and that is what explains the high CPU usage with Firefox. The information that the most impactful event originates from calls to VirtualProtect was forwarded to us by Microsoft, and I confirm it. In Firefox, disabling JIT makes MsMpEng.exe behave much more reasonably, as JIT engines are the source of the vast majority of calls to VirtualProtect.
> On Firefox's side, independently from the issue mentioned above, we should not consider that calls to VirtualProtect are cheap. We should look for opportunities to group multiple calls to VirtualProtect together, if possible. Even after the performance issue will be mitigated, each call to VirtualProtect will still trigger some amount of computation in MsMpEng.exe (or third-party AV software); the computation will just be more reasonably expensive.
> It mentions that Firefox writes wayyyyy too much in the temp folder.
> > the amount of calls to VirtualProtect is currently very high
Calling VirtualProtect is not writing to the temp folder. The VirtualProtect call is to change the permissions of the in-memory pages. It should be an inexpensive system call (other than the cost of TLB flushes and/or shootdowns).
> With a standard Firefox configuration, the amount of calls to VirtualProtect is currently very high, and that is what explains the high CPU usage with Firefox. The information that the most impactful event originates from calls to VirtualProtect was forwarded to us by Microsoft, and I confirm it. In Firefox, disabling JIT makes MsMpEng.exe behave much more reasonably, as JIT engines are the source of the vast majority of calls to VirtualProtect.
> On Firefox's side, independently from the issue mentioned above, we should not consider that calls to VirtualProtect are cheap. We should look for opportunities to group multiple calls to VirtualProtect together, if possible. Even after the performance issue will be mitigated, each call to VirtualProtect will still trigger some amount of computation in MsMpEng.exe (or third-party AV software); the computation will just be more reasonably expensive.