I think what they mean is that the addon doesn't do anything unless you opt-in. Still, why install it at all?

The installation of the add-on is unquestionably a change to Firefox. Whether the add-on does anything is largely immaterial.

Really poor choice of words here from mozilla. "The changes that already happened will not make changes to your firefox unless you enable them to", in other words the changes already happened and are just waiting to be activated.

It's installed, but apparently doesn't do anything if you don't have "studies" enabled.

Something was installed on my system without my permission, from a vendor that I chose specifically because they promise not to do things like that.

To put it another way - if I discovered a rootkit sitting in the ~/Downloads directory on my Mac, that would be a problem. If the entity that surreptitiously placed it there said that I shouldn't worry about it because it hadn't been run, I wouldn't be inclined to trust them.

That lack of permissions make this illegal IMO. CFAA in USA and CMA in UK both make unauthorised access and unauthorised modification of a computer a crime.

I've used FF since before it was FF, and I've installeded it on umpteen other people's computers; strongly advocating for it. Since they sacked that guy for not conforming to a specific liberal ideology they seem to have gone batshit crazy ... what happened? Was he their main privacy advocate or something?

Hahaha. You gave them permission when you agreed to their TOS. But you didn't read it, did you?

ToS terms that aren’t either expected by the user, or explained in plain text without having to click through anything, are considered null and void in the EU.

By that definition, this would be unallowed modification of the computer of the user, and fall under the various hacking acts.

The more interesting thing is that this has also been rolled out to german government computers, which mostly used to use Firefox, but due to previous troubles with CliqZ and the Google Analytics in the addon menus, have already moved on to other browsers.

>ToS terms that aren’t either expected by the user, or explained in plain text without having to click through anything, are considered null and void in the EU.

You mean like the TOS and EULA you agree to when you install the browser? That would qualify as "expected by the user" and "explained in plain text" both.

Correct, those ToS and EULA are considered invalid, if they contain unexpected agreements, such as "your firstborn belongs to us", or "we can install whatever software we want on your system".

Except they didn't install whatever software they want. They said they can install software to check metrics and more. Which is what this is.

"and more" == "whatever"

Check metrics !== advertise a game.

Installing extensions is not "No changes will be made to Firefox".