As other reviews will tell you, the limitations of the new extensions format prevent this add-on from functioning how it used to. Now it just creates two side-by-side windows. Windows has similar functionality built in, yes, but honestly, I find Tile Tabs WE handier than that. Nice way to compare things side-by-side quickly, easily, and conveniently with no fuss. It would be far better if it worked like it did before, but it's not useless. I don't use this extension a lot, but when I'm in need of it, it's still handy. And as we all know, it's the cost of moving to WebExtensions. While I certainly think this add-on is better than the high amount of 1-star reviews indicate, I hope people's dissatisfaction nudges Mozilla to work on a WebExtensions API that enables the old functionality.

If you're REALLY hankering for two tabs side-by-side in the same window, Vivaldi has such a capability...though it's implemented much differently from Tile Tabs and I must confess that, from my brief testing, I'm not fond of their version of the feature.