OMG, just what I needed. There was an old plugin which could not be continued because of changes in Firefox policies, I missed it so much!
Title of group can be changed with double-click on title.
A tab can be moved to a group with the context menu (right mouse).
A new group can be created with the + button in the "panorama" view accessible by the button in the menu at the top.
I did not find a way to move several tabs at once to a, but this is a minour inconvenience for me.