Well with the help of your fiddle(which I briefly copied to my site to rule out some arcane server configuration mistake), I was able to solve it. Totally undocumented behavior and definitely a bug with Firefox, but basically it seems to not like having scripts that come in before the importmap. I moved my multiple CSS links, meta tags, etc. down underneath the shim/importmap and now it’s working fine.
It’s not a specific line that did it, since I could split the CSS files up before/after whichever way I wanted and still get the same behavior; looks like Firefox just has a timeout and turns the feature off once it’s waited too long.
Anyway, thanks everyone for taking a look and helping to work out what was happening. 