I use the Tree Style Tab addon in Firefox for exactly that purpose. The addon doesn't have the concept of named groups, it is just an extension to organize tabs in a tree... but that's more than enough.

Starting with a keyword search on the parent tab, all subsequent links are opened in child tabs that can be expanded / collapsed. When collapsed, the parent tab's title are the keywords, so it's a nice visual way to have the whole sub-tree "filed" into a single named entry.

There are even a couple extras that allow me to long-middle-mouse-click a parent tab, and it unloads from memory the whole sub-tree. It's a pretty neat addon!