That and a no-cache refresh of the Discourse page (CtrlShiftR in Firefox) is all that’s required, actually; and the refresh is only for the impatient, because after reducing the MTU size, while FF is still trying to connect, it will eventually connect with the changed MTU in a retry.
And yes, I’m also affected by this; I only need to do the reverse (set MTU to 1500 again) and FF gets stuck trying to connect, currently. I occasionally do that to see if the issue is resolved, and it works sometimes, so I’d categorize this as intermittent.