As you demonstrated in your post, telemetry's main purpose is to advance what you already want to advance. Your add-on example is a good example. There's no reason to take a widespread add-on and re-develop it as new built-in functionality. It's already working for tons of people as an add-on. Making a new version means you've now split that market, you probably introduced bugs, your implementation will be slightly different, and the features may conflict with each other when the browser automatically updates. You took a working solution and polluted it making everyone's life a little bit harder. Splitting that market might results in both versions failing in various ways. The add-on developer will view their idea as stolen, see a massive drop in users, and give up because they know they can't compete with a company. The browser will see not as much usage as they hoped (since half stayed with the add-on) while seeing people leaving that now dying add-on. They'll clam users no longer care about that feature set and remove it from the browser in the name of efficiency. Now the add-on is dead and the feature is gone from the browser. It's a fail-fail for everyone.
A better response would be asking if that add-on needed any new APIs and double-checking that browser upgrades won't harm it. Having an API means other people can make their own add-ons with similar but different feature sets. If you hate that all the tool/menu/tab/status bars are less flexible than their Windows 98 versions then you could simply swap a different add-on in for those features. Instead with those things integrated, their APIs are removed and you're stuck with whatever bullshit changes companies push out for marketing's sake. The company forces you to follow whatever fad they're afraid of missing out on rather than letting you keep whatever setup you decided was optimal for you.
A clear example of Firefox's telemetry abuse is their change of the Open vs Save download feature. Selecting Open used to mean the file was downloaded as a temporary file, opened by the default application, then deleted when you closed that application. It was pointed out that other browsers do it a little differently and telemetry showed the Downloads Preview Panel usage wasn't as high as they thought it could get. Firefox changed it's Open operation to mean Save and Open. When you close its application, the file is no longer deleted. This unwanted behavior forced people to use the Downloads Panel to delete that left over file. That massive increase in panel use increased it's telemetry and Firefox devs padded themselves on the back while proclaiming the increased metrics meant people really enjoyed the change. In reality they made their download system far less efficient. You now have to perform multiple actions at different times to accomplish something that used to only take one action. If you don't remember to do those multiple steps you end up with tons of poorly named files in your Downloads folder. You now have to go through all those files and figure out which statements you were just glancing at and which ones you actually meant to save. (If this feature was an add-on, like the original Firefox promised, people could have selected which download style they wanted and everyone would be happy.)
Also, a segment of users disabling telemetry isn't a valid excuse for anything. People are supposed to have brains, not blindly take any metric at face value. Telemetry is good at figuring out if/when something breaks. It's poor at determining what users want/need/enjoy.