Tried because Onetab don't have a dark theme. TabMerger does what says, it's more kind to my eyes and make me feel more calm because there's options to undo mistakes and save my precious tabs. But I think it's over complicated, sometimes more is less. Some buttons don't need to appear or some information have a large size and isn't that important (like the day with seconds a group was created). One of the things I loved about onetab was the simplicity. Just when I move my cursor next to a link I see a X showing I could remove or how easy was using the extension without the need of a tutorial.

I came from OneTab, and at first really really liked this. But every once in a while, it deletes all my tabs for no reason. This isn't a huge deal the way I'm using it, but it's something that would be nice to fix.

I use Firefox on Pop!_OS (Ubuntu based) with the following extensions: Beagle Button - Sustainable Shopping, Dark Reader, Don't touch my tabs! (rel=noopener), Grammar and Spell Checker — LanguageTool, IPFS Companion, Kutt, NoScript, Plasma Integration, Reddit Enhancement Suite, TabMerger, Terms of Service; Didn’t Read, uBlock Origin

TabMerger is a very handy extension, but it has one big drawback. Let me explain what I mean. For example, I created a group of 3 tabs called Forums. On the first tab of this group, I added a forum with 8 pages, on the second tab, a forum with 30 pages, and on the third, with 50 pages at the time the group was created. One day passed and I opened a created group called Forums to read the news. During this time, the first tab instead of 8 pages began to contain 11, the second tab instead of 30, contained 32 pages, and the third instead of 50 began to contain 55. I read all the forums until the last updated page, ie up to 11, 32 and 55, respectively. The next day, I reopen the forums tab group, but my reading progress remains at its original level, i.e. 8, 30, and 50 pages respectively. It is necessary for the created group to automatically update the current state of the read forum pages and changed tabs within itself. At the time of creating the Forums group, I had 8, 30 and 50 pages in the corresponding tabs, and a day later I read them to 11, 32 and 55, respectively, it is necessary that when I open the Forums group, I see pages 11, 32 and 55, which I was reading, not the original 8, 30 and 50 when the group was created.
Sorry for my English, as I wrote through a translator.