As I recall it was only enough of the next tab to make it "clickable", but I took the behavior for granted for so long I couldn't say for certain now. The important part was being able to click each tab in turn, from left to right, and so select every tab on the bar all the way to the extreme right-hand side...or the right-most tab.
No, I don't think so. What exactly happens when you click on the right-most visible tab depends on the combination of the (min) width of the tabs you've set, and the current width of the browser window. AFAICS anyway.
I have my older FF 3.6.28 and TMP 0.3.8.7 running now and it behaves so that whenever I activate the right-most tab, it auto-scrolls one whole
tab to the left, showing the next tab in full (my tabs are really narrow in this scenario, 25 pxl). The overall visual effect is slightly confusing actually, owing much to the scrolling not being smooth in any way; it's so fast it's like an instantaneous "jump".
You may know this, but a tip: instead of clicking on the tabs from left to right in turn all the way to the end, press Ctrl+PageUp/PageDown (assuming you're on MS Windows).
Heck, I'd settle for even 25% or possibly even less...depending on what the tab-width is set to, of course. The key here is that the portion of the partially displayed tab needs to be enough to easily click.
Well, 25% of 25 pxl (my min width) is only 6 pixels... That's nasty.
It would be possible of course for onemen to make this progressive such that if the current tab width is very small, it cranks it up to 50%, when they're wider it goes to 33%, then 25% then 10%. I think I'd make it like that so it can look nice(ish) in the majority of cases at least, there are so many combinations of tab and browser window widths possible.
Thanks for the opportunity to contribute, TMP really is one of the very best, oldest and most used extensions for FF. I for one cannot live without it and any power-tabber out there feels the same I'm sure.