I'm so impressed by this add-on that I had to share some praise.
This is by far the best tab manager/ bookmark manager I've ever used on Firefox or any other browser.
It functions like One Tab, Group Speed Dial, and a bookmark manager all in one.
Everything is intuitive and every icon does what you think it will do. The pinned tabs are horizontally laid out above the vertical list of the all of the unpinned tabs. You can create a group from those tabs and nest the pages. You can expand and expand the groups of tabs and create a new group. Everything has quick access. You can drag and drop just about anything. It's my pick for the Best Productivity Add-On of the Decade. And that's coming from a real add-on junkie. Thanks devs and or devettes!

Finally found the one. Keep up the good work, this should be default in firefox, and for sure more people will see what they are missing. The amount of tabs that I can have open and organized is just mind blowing.

Edit: Still going strong in 2024

Love it.

At a first glance, one might be tempted to say that this is just another Tree Style Tab clone. And without many users, why even try it, right? But one couldn't be farthest of the truth by that.

Having tried Tree Style Tab and Tree Tabs, this is far the best of the three, but by a large margin. Why? Because here you feel you always know how to make the addon work for you. Does it look great? Indeed it does, but so do the others. Do tabs become well organized, easy to find? Yes, but to some extent so do they on the others. Is it stable? Indeed it is!, but so are the others for the most part.

Where Sidebery shines is in its settings. For there you feel you are in control. Every option is clear, well organized and aimed at some user case that might suit your particular needs — even those you didn't think you had.

All in all, Sidebery is a great addon, stable, sane, easy do use and configure. And in this particular arena of Tree Tabbing, it's also clearly the winner. So, should you try it? Most definitely!

This add on is faster than other vertical tab manager.
Very promising, it just lacks the coloring of the tabs by group for me to adopt it.

Has the most options of any addon I've used!

Very good addon with possibility to hide your tabs that is good in tab-management, beatiful UI, but multi-panel doesn't work properly. Sometimes tabs dissapear from a certain panel, hope you will fix it.

p.s. Is it possible to change ONLY group icon color?
In example, "favicon-placeholder-bg" selector does not fit to this case very well, because it changes other icons aswell, but in order to visually separate tabs this thing could be very useful

p.s.s. Found solution if someone needs. Place it in Style edit.

.Tab[data-parent] .fav .placeholder > svg {
fill: red;
}

But it changes all your data-parent tabs, remember this.

If you need to change color/text-align of your groups in tab panel, try this:

.Tab[data-parent] .title {
color: black;
text-align: center;
}