I hadn't tried this Tab groups feature yet, but in a relatively clean profile, it seems reasonable with the Light and Dark themes on Windows (screenshot attached).

Maybe the Container line is the problem? Well, again, it seems okay in the built-in themes for me.

So maybe this is a problem with custom/add-on themes, or a difference between PopOS and Windows. Those themes might need some color tweaks to make sure the line is fully visible against the background?

Users of a hackerish persuasion can use custom style rules to change default colors, line thicknesses, etc., but since this feature may be getting bug fixes for the next few months, it might make more sense to raise the issue with your distro maintainers to find out whether it is something they will tackle or it needs to be escalated to Mozilla, or it needs user tinkering.

Also, if it's not considered a bug, Mozilla has a product suggestion site at https://connect.mozilla.org/. If you have time, I think it would be a good idea to post this request there to get into the current feedback stream. Use the "Ideas" section of the site (at https://connect.mozilla.org/t5/ideas/idb-p/ideas).

I tried changing the theme to the light version, became immediately better to see!

I'm not keen on modifying themes anymore. I used to be big on modifying such small details, but everything would became a hassel...