"I'm not sure what their reasons are for experimenting with something new but I would be curious to know." This is pure speculation on my part, but I assume th

"I'm not sure what their reasons are for experimenting with something new but I would be curious to know." This is pure speculation on my part, but I assume the "Firefox team" wanted the selected tab to stand out visually. The selected "tab" turns white in the default theme. But if the tab is attached to the URL toolbar, then the URL toolbar also needs to be the same colour as the selected tab. It would look odd if they were attached but each was a different colour. I assume the "Firefox team" wanted the active tab to stand out visually, but not the whole URL toolbar. So the tab becomes a button and is detached from the URL toolbar. Now the active "tab" and the URL toolbar can have different colours: A "louder" colour for the selected tab to make it stand out, and a "quieter" colour for the URL toolbar that lets it recede a little from the foreground. That's what I think may be the reason for the tab change - but as I say, it's pure speculation (and could be completely wrong).