I'm using Firefox 48.0.1 and I just booted Firefox up and I got a warning from Avast about a addon with a poor rating, the exact screenshot:

enter image description here

I don't recall installing this addon and I can't find it on the official Firefox addon store.

Does anyone know what this is and if I should remove it?

  • bugzilla.mozilla.org/show_bug.cgi?id=1268197 - remove it if you don't need it Aug 24, 2016 at 10:39
  • Yeah I seen that but wasn't 100% what it was and if I needed it lol
    – Brett
    Aug 24, 2016 at 11:26
  • Solution: (I am dead serious) The only malicious software you have on your system is Avast. Avast was poorly designed to look at this stuff. You should disable this feature immediately.
    – Ramhound
    Jan 20, 2017 at 0:04

2 Answers 2


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!

  • is there any user-facing method to view and/or manage these "system addons"? (about:systemaddons or something?)
    – quixotic
    Jan 20, 2017 at 0:14
  • Not at the moment. There are some discussions (sorry, unable to find the bug number right now), but I don't think we need to have an UI for this. Some system extensions, like Pocket, can be disabled inside the UI already, for example by removing the icons from the toolbar. The webcompat addon also will have a way to disable the overrides for web developers. In general, if an addon does not provide an UI for disabling it, there is a reason not to disable it. (That, however, is just my opinion. We'll see how the discussion goes!) Jan 20, 2017 at 1:07
  • Very interesting - thanks for the breakdown @DennisSchubert
    – Brett
    Jan 20, 2017 at 8:17

Today I had the same Avast warning: Mozzilla announced in July 2015 to detach part of Firefox's core functionality into so called system add ons in order to go faster. Different from user add ons, to adapt Firefox to your user needs, these system add ons are not managed with the Add On Manager. Not beeing a developer and using Firefox as my browser I won't mess around with system functionality. (And certainly not based on an anonymous rating mechanism of AVAST which does not even pretend to have identified a technical risk and just argues 'do what everybody else is doing!') Im sorry that the source I can refer to for further reading is in German. https://www.camp-firefox.de/node/1405

I won't allow AVAST to remove part of the system functionality of my browser, and would nobody suggest to do so.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.