Aangaande dié byvoeging
Other features available are "Merge Windows" which moves all tabs from all windows in to one single window, and "Detach Tab" which makes a window out of a tab.
For both features you can select tabs with Ctrl+Click. This allows you to merge only the selected tabs into a window, or to detach all selected tabs as tabs in a new window.
All actions can be invoked by toolbar buttons, shortcut keys and context menu items.
Because I notice that people still have problems with the keyboard shortcut , I will state here: please install keyconfig extension (http://forums.mozillazine.org/viewtopic.php?t=72994) to rebind your keys. Keyconfig will also show you when a key is already in use by either Firefox or another extension.
For adding the toolbar buttons look at my home page: http://twanno.mozdev.org/duplicatetab/#toolbarbuttons
You can go there for other problems as well.