This is my first SDMB post; I apologize for the likely errors.
The OP appears to be running Firefox 115 ESR on a Mac, perhaps running MacOS 10.13. (Apologies if this has been stated explicitly; I haven’t read all the posts). I’m in the same boat, and received the same banner message. I’ve looked around for browsers that support MacOS pre-10.15, and stumbled onto Firefox Dynasty. This is a project run by someone apparently out of love (or lunacy, I suppose) that takes recent releases of Firefox and ports them backwards in operating systems. I’m typing this using Dynasty 136.0.3 and MacOS 10.13.6, and it’s working perfectly. And no banner, and according to the Discourse Feature Test, all three new Discourse features are supported.
I’ve only been running Dynasty for a few days, so it may have bugs I haven’t encountered, but it might provide the OP with an alternative to purchasing a new Mac. YMMV, but I hope it doesn’t. I’d be interested to know if the OP or anyone else tries Dynasty, and what they think of it. PS I am not the potential lunatic referred to above.