Originally, i too, have thought that Mozilla's telemetry would be limited to only technical aspects, such as crash reports. I mean, they are using privacy as their main selling point. But of course, that was a lie. Read this. You may think that only Spyware Watchdog and Dig Deeper dislike Mozilla, but no. By searching around, it's possible to find information about this on countless sources. And again, why do projects like LibreWolf and Arkenfox exist, if Firefox's tracking is not an issue?
Firefox makes unsolicited connections on startup, uses Google Analytics, and connects to their website when opening every single page. They are not leaking all of your data, like Opera and Chrome do, but such behaviour is very concerning. Why do they need to know how many times i have opened my bookmarks, or when i cleaned my browser history? Extremely suspicious, and on top of that it makes the browser a bit slower.
I believe that what happens on your PC, should stay on your PC, the pages you open in your browser, stay in the browser.
It's probably due to Invidious and Piped instances getting rate limited. LibreTube uses them as the source of the videos, i think.