Just for S&Gs, see what happens with Waterfox.

I tested, the same result, nothing changed, still no IPv6 to use

I'm just curious now... test this. http://waterfox.net/

Working fine, I checked and it's working without any issue

Interesting...

Open a bug and provide a link to this thread and definitely mention that it works in Waterfox.  ;-)

it seems many peoples have this issue as well https://www.reddit.com/r/firefox/comments/g4yjhh/firefox_has_real_ipv6_address_but_avoids_using_it/

This Bug link https://bugzilla.mozilla.org/show_bug.cgi?id=1878445

You can also ask the dev team @ Waterfox. If they're nice guys they may help.

Good Luck... Jones out.

https://www.reddit.com/r/waterfox/comments/1ahjwl8/ipv6_not_working_in_firefox/ I did

Post screenshot comparing Fire and Water in that reddit post.