There is no plugin for Tab groups, it is a built-in feature of Firefox 4. If it is the tab groups button that you need, control-click on a toolbar and select Customize. This will open the customize window dialog, you can then drag and drop the Tab groups button onto the toolbars.

https://support.mozilla.com/kb/How+to+customize+the+toolbar

I was talking about the extension/addon

https://addons.mozilla.org/en-US/firefox/addon/tabgroups-manager/

As it works in beta 12, it will very likely work in the release candidate. You can bypass the compatibility check which will enable this extension by using the Add-on Compatibility Reporter extension- https://addons.mozilla.org/firefox/addon/add-on-compatibility-reporter

Thanks! For now it works!