This has been my favorite vertical tab solution for a very long time (and this review is long overdue).

How I use it: In TCR options, enable "Use current browser theme", "Compact Mode", "Favicon-only Pinned Tabs", and "Custom Stylesheet". In the stylesheet, in addition to some tweaks provided by the author (thinner scroll bar, hidden header-- see the link in the documentation), I also add "#topshadow,
#bottomshadow {
box-shadow: 0px 0px 7px 5px rgb(30, 34, 40) !important;
}" to darken the overflow shadow within the tab list. This color is very subtle and matches my favorite them, GitHub Dark Dimmed by evgnz.

Mozilla is finally working toward adding built-in options for vertical tabs, and I'll definitely be judging that implementation against this wonderful extension, as well as the great vertical tabs offered by Vivaldi.

Although I used Edge for a while, I returned to Firefox after hearing about Microsoft's plans to move on to Manifest v3. If there's one feature that I liked from that browser, it was its vertical tabs, which are a more efficient use of screen space.

Going back to horizontal tabs felt like a regression, but this add-on almost brought vertical tabs TO Firefox; After hiding the tab list and replacing it with the title bar, Firefox feels somewhat more like Edge, which I find pretty neat!

Edge-to-Firefox migration story aside, this add-on is a good use of Firefox's sidebar, turning it into a list of any tabs that you have open. It looks nice, adapts to your theme (including ones made using Firefox Color), and at times feels like a natural part of Firefox itself.

Like another reviewer mentioned, tabs can't be grouped with this add-on, which is sort of bad, but I didn't use tab-grouping often in Edge, so not much is lost here.

In conclusion, if you prefer tabs that are listed vertically, are coming from Microsoft Edge, or want to experiment with a new UI layout, I recommend Tab Center Reborn! I've only used it for less than a day but it's mostly exactly what I was looking for.