We're seeing a spike in Firefox crashes, caused by Avast anti-virus

They've had a way to blacklist loading DLLs for a very long time [0], by patching the call that loads DLLs.

That rather basic approach got a little bit more sophisticated in 2019 as well - following an approach that Chrome took.

[0] https://bugzilla.mozilla.org/show_bug.cgi?id=524904

Load up “about:crashes” and make sure at least one of the crash reports was submitted. Then click the link to view the report; it’ll show you the stack trace, what libraries were loaded, all kinds of information. You can use that, in combination with the Firefox source, to diagnose the crash and figure out why it is happening.

I mean when you weigh pros and cons of using 3rd party AV software, that's a pretty reasonable idea.