No, I've heard of people using it for years. They probably just moved it to make it more visible

The profiler has been there for ages. It's definitely not new. They're moving where it's accessed but it's been under the Performance tab of the developer tools for a long time.

Ah, I'm aware of the feature in the devtools, but I thought this feature was designed to capture the performance of the browser UI itself.

That also exists, see https://profiler.firefox.com

I am sure Mozilla would appreciate a bug filed in Bugzilla if you can.

There are two different profilers. There was one in devtools for a long time that was aimed at web content. It was pretty nice for its time, but its time was a long time ago. In a separate project, a profiler for both content and browser UI was developed—that's the one at https://profiler.firefox.xn--comand-5g0c it has been seeing a ton of high quality work for quite a while now. At some point, it was clearly so much better that they started enabling it to optionally focus on web content (though platform stuff is still accessible if you need it). The recent change is the old devtools profiler has been put out to pasture and replaced with the newer one (with presets configured appropriately for web content), and additionally the platform profiler is more integrated so you don't need an add-on or anything; going to https://profiler.firefox.com gives you the option to activate the built-in functionality.

The Firefox Profiler is an amazing piece of work. It's my tool of choice even for non-Firefox scenarios, since it can load in perf profiles (and even Chrome profiles, though I haven't tried that). It has grown into a general profiler front-end, and continues to see significant improvements. I have heard of a number of people adopting it as their system profiler of choice for arbitrary workloads, not just browser-related ones.

(I work at Mozilla, but not on the profiler. At least not most of the time. I've added some small things in to give it more information about my specific subsystem. I have enormous respect for the work going into the profiler, both the functionality and design.)