I think AI in the browser could be useful. It just isn't that useful now.
So far, the most useful "AI feature" Firefox has ever shipped is the page translation system, which uses a local AI to work. I wouldn't mind seeing more of things like that.
Eventually, "browser use" skill in AIs is going to get better too. And I'd trust Firefox with an official vendor agnostic "AI integration" interface, one that allows an AI of user's choice to drive it, over something like OpenAI's browser - made solely by one AI company for its own product.
Yeah I use a plugin for similar translation functionality, but with a local llama.cpp instance instead. Definitely useful and has increased my usage. Also works nicely on the Android version of the app.
Firefox is only good for getting forked into better browser like Mullvad Browser, LibreWolf and Tor Browser.