I've tried Sideberry and TST, but tend to prefer Simple Tab Groups[0] to either of them. The advantage is that STG reduces the clutter of tabs on the top-pane and of course allows one to group them and move them about. Sure, Sideberry allows you to group them and it shows the grouping in the side-pane, but all the tabs still show up on the top-pane.

[0] https://addons.mozilla.org/en-US/firefox/addon/simple-tab-gr...