Safari was late until a couple of years ago when they started implementing new features more aggressively. Now it's always Firefox. Just check how long it took them to add support for the :has() selector and RegEx Lookbehind. We're years into "manifest v3" and background workers are nowhere to be found.
Firefox has mindbogglingly bad performance with :has() even if their superficial tests tell you otherwise. I experienced minute-long lockups just because I used :has() on a large element like body. Chrome and Safari had no issues with those selectors.
