Full disclosure: I am a Mozilla employee and in fact one of the developers working on the add-on in question!
It is true that we're now releasing some features internally as something we call "System Extension", which technically is the same as an addon you could install on addons.mozilla.org, with the difference that these come pre-installed with Firefox and there is no way to disable them. We mainly do this to be able to ship updates faster, but it's also nice that we have some features totally separated, which makes the development process easier for us!
I am highly confused that Avast claims our system addon is evil. The version of our Webcompat addon that is currently included in Firefox does absolutely nothing: it's just a placeholder for us to replace once we have built what we are building. I'd really love to know how their "proprietary analytics" looks like and why we get rated that bad. However, I can assure you we don't do anything evil! :)
I highly doubt Avast is even able to disable/remove system extensions, but even if it is, you absolutely should not do it. Some system extensions may actually have dependencies into others, and stuff could break horribly if one or more of those are missing, so please don't remove them.
As for the addon itself, let me explain what we plan to do. The description already says "Urgent post-release fixes for web compatibility" and that's pretty accurate. The WebCompat team at Mozilla ensures that the web works in Firefox, basically. Sometimes, we release a change inside Firefox that may break specific websites and we need to do something about this. We use this system extension to ship such fixes whenever needed. If you want to, feel free to read more about our Web Compatibility efforts in our wiki!