AV causes all sorts of problems for Firefox, such as startup crashes or, in some cases, AV MITM breaking Firefox updates, leaving users stranded on old Firefox versions. In Firefox 53, Mozilla is starting to purposely make it more difficult for extensions to rummage around in native code and external software to inject DLLs into the running Firefox process:

https://blog.mozilla.org/addons/2017/01/24/preventing-add-on...