Just installed today's RC2.
Firefox couldn't connect to local devices (again) until I toggled off/on the relevant access in Privacy settings.
What a bummer!!
But quite frankly, I think it has gone for me in 15.1.1 now, with
• I used the 2.5 G Ethernet port of my Dell U4025QW monitor the whole time, getting its Mac mini connection through the TB4 connection
• Having to try something with the Mac mini directly connected to Ethernet, I swapped the cable around and plugged it directly into the mini, not even testing this issue, as it was for other reasons
• Plugging the Ethernet cable back into the Dell's 2,5G port again, things seems to be fine for the last four days
Go figure - but I just hope it won't come back with 15.2 now
I'll definite know as I'm using MQTT Explorer on a daily basis.
And as @brewmoney rightly mentioned before
"It's that dreaded worst sort of issue - intermittent."