EDIT: found a suitable replacement for Séparé at UserStyles.org call "Tabs - add dividers like Séparé." It duplicates the seperator functionality nicely (though not the preview functionality) and works with Tree-style tabs. You'll need to install Stylish first or be comfortable pasting text into your Userchrome.css to install it.
In the past, I'd used Separe with TabMixPlus, and it was ok. The one major problem was it took too much real estate (i kept my tabs fixed-width, so that's how large the seperators were), so it got uninstalled.
I recently installed Tree-style tabs, a vertical tab bar variant, and thought about installing Separe again. If anything, Separe would be better suited for a vertical tabbar environment as you lose only the height (not the width) to the divider. I get ~40 tabs vertically, so sacrificing a few to a divider would have been no problem.
Unfortunately, when a new divider is placed, the tabs' width shrinks down to Separe's (narrow) width and severly compromises the tabbar's utility.
Not blaming this on Separe as it can't be expected to work with every plugin (and in fact Tree-style tabs may be the culprit, as it has plenty of bugs on it's own), but it's too bad it doesn't work in this setting.
This add-on is actually a 2-in-1 deal. First it makes separator tabs in which you can organize your open tabs with. So say you have news and sports tabs open and you want to organize them. So just open a "Separe" tab and move all the "News" tabs to the left of the "Separe" tab. There you go.
Also Separe also doubles as a form of tabbed browsing! Just click on a "Separe" tab and you're treated to thumbnails of all your open tabs and options on how to deal with those tabs (including switching the view to any open tabs you have currently). This add-on is very useful. This gets a 10. Great add-on!