As a recent Chrome uBlock Origin refugee, I was looking for something that'd replace chrome's tab groups.
This add-on does a little bit of that, but differently, and in several ways better.
First off, you WILL want to customize your browser toolbar to put the add-on icon to the top left of the app, right before the tabs. It's a much more natural place to use it.
Second, if you're using dark mode, the default icon is hard to see, and you'll have to tweak a config to fix that. It's spelled out in the "about this extension" page.
Third, if you multi-screen, this is MUCH better than chrome tab groups, since it allows you to trivially move tab groups between windows, and therefore screens.
I like that it can sync the tab groups into a bookmark folder.
The default group icons are awkward, but you can replace them with the favicon of any tab within the group, which solves that.
I just installed this a couple hours ago so I'm still wrapping my head around it, but I like it already.
I've got 12 tab groups already. Just don't expect it to be a drop-in replacement for Chrome tab groups. Take the time to wrap your head around the mental model of this add-on, it's worth it.

*Edit, now with more gripes!:
A dozen tabs or so (about 8% of my tabs) get detached from their group on every Firefox restart, which means on every Firefox update for me. I don't know why. It's not the same tabs each time. I tweaked all the settings I could, didn't help. I have to reassign the stray tabs in their proper groups after a restart, which takes entire seconds.
I'm kinda hoping this is going to stop happening on a future update. If I was a little less lazy, I'd probably try to debug it myself.

Essential plugin. Way better than Chrome tab groups. Does everything I need

BRILLIANT! Simple, intuitive and classy. Open, create groups, drop and drag tabs between groups, then choose any one from the icon on top at any time.
I figured it out in 10 seconds, made 3 groups in 15 more, filled them in 20, and now just have to click their icon up top, and choose one of the three groups (which you can name, and which have a little color on each one).
This is a case of programming perfection: it could not have been made better!
Change colors and change icons or add your own in several common formats - download anything. Nifty!
I even grabbed a 1200x1200 pixel jpg off my PC and it worked, and shrunk it perfectly. Most websites choke on 128x128. Dude! :)

Un pur plaisir à utiliser ! Si vous aimez grouper vos activité, et économiser de la RAM en pensant de l'une à l'autre, alors cette extension est juste parfaite !

As I often work on different products and different release versions, I love to now have the possibility to create groups based on product/versions. I used to have all important Jira tasks in one window (as I do not like to have several Firefox windows opened at the same time), but thanks to this plugin I can now have them divided in different groups and switch between them easily. Pinned tasks stay visible allthough I am switching between groups which makes this plugin even better! Thank you Drive4ik!

it's perfect
me <3 containers

I love it. Took me a few minutes to figure it out especially connected to container tabs, but I cannot do Firefox without it now. Thank you!

The backups of this addon saved me all my tabs when the shitty Firefox 127 update decided to nuke my whole history/open tabs etc without a way to restore it. Just trusty old STG churning out backups which I could restore from. Just for that the 5 stars are justified, but on top of that it also is a very useful addon.

Great add-on. Coming from chrome it takes a bit of getting used to, as it isint as polished and intuitive. I really do only have one problem with it tho:
For some reason you cant specify where the backups folder is placed. It just defaults to where firefox's downloads folder is. And i dont want this backupfolder to be in my downloads. plz fix.

If you have more than five tabs open or more than two windows to separate your tabs thematically, do yourself a favor and install this add-on. If you're a keyboard shortcut lover, set them up and enjoy the organization of your tabs taken to the next level.

So far 5 stars, but I haven't used it that long.

It allows me to create groups like in Chrome, but unlike Chrome, these simple tab groups replace the all the previous tabs you have open (when you switch to a different group).

Other extension open them in a new window and load them all, which I don't want, I mean I have groups with 1000+ tabs in them and loading so many can crash weaker computers or cause certain websites to block you.
Others like Chrome open the new tab group in the same window making it cluttered and unorganized.

Your ungrouped tabs are not lost when switching to a group, but I still recommend creating a group for them, just call it 'casual' or 'random'.

And unlike new windows or Chrome tab groups the 'vacation' or 'job hunting' group you weren't supposed to be looking at at work can be quickly switched to/from and perfectly hidden.

I was afraid it doesn't have backup options, but thankfully it does.

This plugin and Download Manager (S3) made it so I can pretty much switch over to using Firefox instead of Chrome, and that's pretty huge.

очень простое, но полезное расширение

Lot of customization options & a bit confusing to use at first, but once you figure it out very convenient! This will be a great hold-off until the Firefox team offers a native, built-in feature.

great addon. thank you so much for creating this : )