Vertical tabs and tab groups (I suspect it can't be that different from folders but I could be wrong) are available in Firefox.

And they're really good. I'm so glad I've discovered this paradigm.

I use a vertical task bar on KDE and a vertical task bar on Windows at work. It's such a huge productivity boost. First, I can see WAY more window previews at one time than before. And second, I can use text to tell the windows apart. 5 Excel workbooks open? No problem, they each have a name. No more clicking on one icon and then squinting at window previews to see which one you need.