Excellent add-on, thanks for the hard work! You cannot set font size and spacing, but the feather theme is very condensed and I like that. I hope this is going to be available on Chrome at some point.
A couple of suggestions:
1) remove the dividers among items, they add weight and are not needed. UI will look better without.
2) I like to keep the panel on the right hand side. When many tabs are opened and a scroll bar appears, it's quite difficult to hit the delete icon. I suggest to make the target area larger.

Good, and (unlike some others)
has a close button for the tabs and
an option menu.
But, settings can't be changed in Private mode
and there's no simple button to switch
between bright/dark mode..

I used to love this but now it has a gear settings icon on top of the tabs with no apparent way to hide that.

Been using this for a week or so. Very nice. Would love if you could add Firefox container options to the context menu when you right click on a tab, i.e., "open in another container...". Thanks!

This is very useful. I hide the top tab bar with a userChrome hack and just use this addon. I like the compactness of the tab bar.
What I miss is support for container tabs - VTR doesn't show the container color and doesn't have the menu to move tabs into containers.

Just installed it and Wow! It is reacting to clicks much faster than Tree Style Tabs. Does not seem to require too broad permissions. Does not come with attempt to show tabs in tree which I anyway disabled whenever I could. Just seems to do well what it should do.
I hope that Firefox stops leaking memory like crazy as it used to do with TST.

I wish it also had bright color theme with less intense colors.

Not really a review, just a question on the add on. Is it possible to move the vertical tabs to the opposite side of the screen?

I like the clean look and it works well for most part, like it has reload all tabs built-in, but it can't select group of tabs to bookmark, the original tab bar in firefox or tree style tabs can, yes u can bookmark all tabs but no selective, I hate the horizontal tabs bar on top, make my neck hurt to look up and down the tabs, my favorite is at bottom of window, but settle on the side with no choice.

I just started using VTR today and find it clean, unobtrusive and easy to use and works well with the native Firefox sidebar. I think it will become my fave tabs manager very quickly.

I use macOS Catalina 10.15.4 and I can't hide horizontal tabs; without this feature the extension is much less useful.

FOUND A BUG! Devs please fix this as this is really annoying! When I open new tab and close it, it does not activate THE PREVIOUS tab, but rather next tab below the one closed! Imagine I google something and keep opening and closing results in new tabs. All the time I need to manually switch back to my google results. Very frustrating.