Let me fire up the ol' rant machine...
Ah fuck it... It's too early in the morning.
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In the great order of things the "megabar" isn't that big of a deal.
I personally hate it, but that's just me.
Objectively it's bad UI for a few reasons, but I don't want to write a big screed about UI design.
At the root of the problem is what dude said above: their attitude stinks.
They activate it without consent.
They remove the option to disable it.
Mozilla has become a caricature of itself. They were already just a fig leaf over Google's hegemony (They get 85%-90% of their money from Google and send telemetry to them by default.) Now it seems like they are adopting the same patronizing and distant stance.
I hate to say it because I used to really like Mozilla and I even have some friends working there so I know it's a bunch of really good, committed folks over there, but to me their progress and process have become retrograde along the dimensions I care most about.