If it's on top, it's the currently active tab. I'm not sure on the black border around white icons, I don't use the light theme. Tab groups are the colored bor

If it's on top, it's the currently active tab. I'm not sure on the black border around white icons, I don't use the light theme. Tab groups are the colored borders, very handy if you use containers. That's one of the killer features to FF, if I'm in a Google, Facebook or Amazon domain I'm isolated. Then when I click an exit link or anything not categorized, I'm in the default container. It's stress-free browsing for me as I got tired of seeing online entities pick up things about my life that it shouldn't have had access to. I also like Safari (and Edge). I've been using FF since it was in beta (Phoenix) and never left, FF is the only browser popular with the features I think balance out the energy efficiency improvements of native browsers.