(In reply to Jim Porter (:squib) from comment #7)
(In reply to jfftck from comment #6)
For me, the first tab is currently fixed to the inbox; this should be dynamic based on which item is selected on the spaces bar.
Assuming this hasn't changed in the years since I worked on Thunderbird, that's an architectural limitation due to needing at least one instance of the 3pane tab (aka the inbox) in order for Thunderbird to function. (Essentially, this is because tabs were retrofitted onto Thunderbird for version 3 all those years ago.) I know there's been some progress on fixing this, but I'm not sure TB is to the point where you can just have no 3pane at all.
You could just hide the tab when not on the inbox, and place a tab for the active space. This is easier than combining the tabs and spaces into 1 bar. Also, the side tabs would have to take up more horizontal space for the title, which doesn't sound like a good tradeoff. Everything is styled with CSS, so it isn't hard to hide the tab when not active but still leaving the 3 pane layout in the state it currently goes into when another tab is selected.
The UX should be to remove having tabs and buttons that do the same thing, this is a start while having the end goal of implementing a fix for the tabs. Having a short-term solution is better than having this confusing double task switching. I know it is a hack, but it sounds like tabs are a hack already.