This extension has finally allowed me to upgrade to Firefox Quantum. I had Tab Groups extension installed which was not supported on new Firefox at that time and didn't want to make a move without that or similar extension. Simple Tab Groups are just what I needed. Allows to group tabs, to switch groups, doesn't load all tabs in group but only active one (saves RAM).

To all unsatisfied reviewers:
* Tabs loss is not due to this extension, this is an acknowledged Firefox bug. The extension author has shown it happens even without his extension. It is registered in Firefox bug tracker. The best you can do for now is to check that the extension is tracking your tabs (colored triangle on extension icon), back up tab groups before closing Firefox, and hope that either extension author finds a workaround, or this bug is fixed in Firefox itself.
* Adding moz:extension scheme to url is the way to avoid tabs reloading when switching groups. If you disable Fast Group Switching, you'll disable url mangling, but you'll notice that all tabs are loaded at once. This, again, is a limitation of Web Extensions API in new Firefox, which doesn't allow to hide a tab, only to close it. And when you switch groups, you'll need to reopen them. I've heard that tab hiding has finally been added to Web Extensions API. Let's hope it's true, and wait for new version of Firefox which supports tab hiding.

Sad to see undeserved bad ratings.