Additional information: I tried FF on another Mac on my network and I was able to load the gui, so there's definitely something borked on my copy of FF.

I just tried a Refresh and that did not work. http is still being forced to https.

I think you're running into Mac OS Sequoia's firewall blocking Firefox from the internet. Read this article: Firefox blocked by macOS 15 Sequoia firewall - What you need to know

I think you're running into Mac OS Sequoia's firewall blocking Firefox from the internet. Read this article: Firefox blocked by macOS 15 Sequoia firewall - What you need to know

Thanks for the input, but I'm able to use FF with no issue on Sequoia. Turning off the firewall did nothing.

The problem remains that FF is forcing my router addresses to https with no way to override to http - and I cannot login to the router using https - in fact when I attempted to set one of my routers to use https it became impossible to login and I had to do a reset that cost several hours of my time.

I've searched for all manner of 'fixes' and none of them work. I did a Refresh of FF but it made no difference.

Yes, I can still login to my routers using Safari, but FF is my preferred browser and it makes no sense to me that one day it worked and the next it absolutely fails after a update.

Oh wow. I totally see what you mean now. Sorry for missing that!

I'm going to see if there's a bug filed for that because no one would expect a local network site to your router to have a SSL certificate.

This bug matches and has a workaround in comment 18: Cannot access local network sites in macOS Sequoia https://bugzilla.mozilla.org/show_bug.cgi?id=1919889#c18

I hope it helps. This is a seriously strange issue I was not expecting to see.