Sidebery is my go to for that.

https://github.com/mbnuqw/sidebery

Definitely makes Firefox irreplaceable for me.

Tab groups are on by default in nightly, and though I haven’t been on the computer much this past week they seem well realised. This article suggests they can be enabled in other versions, but I only really use nightly so I haven’t tried it..

https://www.ghacks.net/2024/12/03/how-to-enable-tab-groups-i...

What's wrong with the multiple Firefox addons that provide this functionality already? I'm not trying to be snarky...I'm genuinely curious.

I've been using Simple Tab Groups [1] for many, many years.

1. https://addons.mozilla.org/en-GB/firefox/addon/simple-tab-gr...

Looks like it's just around the corner. See previous post: https://qqrl.tk/item?id=42499693