I really like this add-on, but now I have a page that tells me: "You cannot use developer tools with this page. Please close them and refresh." …if the sidebar is toggled on. Are there any workarounds?

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..

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!

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.

Works very well. Not sure if it is possible, but it would be nice if windows with only one tab hid the sidebar by default.
update: does not work in Android on my Cosmo Communicator. nothing is visible.

Ookoo...

For every one looking for a solution to hide the horizontal tabs, tested and approved : https://www.hackerway.ch/2017/11/17/firefox-57-quantum-and-vertical-tabs/

I was using Tab Kit 2nd Edition and very much liked it. With Version 52.0 it went obsolete, so I'm using this now, This is pretty nice, but a couple of things I'd like to see:

1. Resolution to hiding the horizontal tab bar.

2. A true autohide of the vertical tab bar that does not require Ctrl-Shift-V.

This is very nice anyway. Would be great to see these things as well.

after loosing Tab Mix Plus i checked this. its good+works. for me the theme fonts are too small@the moment. and i like tree tabs option to save sessions (should be autosave as well)

ok

Great

Pretty useless. There are still tabs at the top taking up room and I still have to go to the top to close a tab. The X on the side tabs don't work. I'm using Firefox 59.0.3.

*** Had my hand slapped for not reading the description. I read that a couple of weeks ago and forgot. I was in a bad mood because I coulnd't close the tab on the side, only at the top. Well, that's working today.

Great functionality but glitchy.

Very nice. :)
Looking forward to future improvements as webextensions itself matures.

Edit:
Found a minor bug. If I drag tabs from one window to another, they don't show up in the VTR sidebar.
Still very nice though! :)

Edit2:
One more minor thing: if you're viewing in Firefox's own Dark theme, the icon is barely visible in the toolbar.

is it possible to hide the tabs on the top of the browser? just like it did before firefox57. thanks!