In a vacuum, it's not. The market is completely dominated by Google though, and they exercise that authority and mindshare brazenly, to the point that it's easy to see how it affects Firefox. Firefox functions great. It doesn't function great with Google products/services such as YouTube in this case, which has no real competition. What's someone to do? Switch to Chrome. One might even make the assumption that the repeat offenses over time, across various services, across various fields, demonstrates... malice.
Not demonstrates, hints at. In each of these cases, Google is very careful that you can easily come up with an alternative plausible explanation. It's hard to use the en masse argument if each of these problems can be accounted for.
These guys are not stupid - they're happy with the current status quo where they have a de facto monopoly but they can pretend they don't.
Sorry, it is absolutely hard-core malice. No hinting, after we found out last year that a deliberate sleep was introduced with a user agent check. This is bald-faced murder. Its like shooting someone in public and stomping on the body. No need to dance around it.
You should actually read the link you posted. That had nothing to do with Firefox and has everything to do with ad blockers. Looks like YouTubes goal was to bring parity between those using ad blockers and those not, as to not incentivize ad blocker usage, not to cripple Firefox.
And ? It is part of the anti-adblocker code which is clearly targeted at Firefox. What sort of extra-ordinary bar are you keeping for malice if not this ?