No it had nothing to do with Firefox writing files. Firefox was making a bunch of calls to VirtualProtect. Windows Defender (MsMpEng.exe) was then writing to file (an sqlite database) every time one of these calls was made, which was slowing down the system.
I can assure you that I perfectly understand the bug and corresponding patch/fix. The patch fixes Event Tracing for Windows (ETW) and how it allocates temp buffers.
The speculation about SQLite at the top of that Mozilla bug report is mostly irrelevant.
This comment is a good summary of what the issue was once they understood the problem: https://bugzilla.mozilla.org/show_bug.cgi?id=1441918#c82