When tabbed browsing was introduced in Firefox, I knew they had something that was indispensible for surfing the web. It eliminated the need for cluttering the OS with so many additional windows, you didn't know where you initially started or stopped.
The idea behind TabGroups consists of taking a common list of tabs and putting them into another grouped tab, organizing your bookmarks even more.
At first, I didn't think much about grouped tabs until I looked over my bookmark toolbar. That's how Firefox is already thinking! I already was starting to collect grouped bookmarks on my bookmark toolbar. Each item in my bookmark toolbar was already considered a group. It started to make sense.
TabGroups is considered alpha software and is pretty functional already for demonstration purposes. This was taken into consideration when finally rated.
One issue was the ability to open multiple bookmarks into one group. I have a couple of web stats sites that I look at often, so I thought I'd add a new group by clicking on the "new group" button on the left hand side and "Open all in tabs." It opened four sites in the first tab group and then proceeded to add another tab group and added the remaining two sites into the second group.
This is considered minor and may have been a theme issue as opposed to an extension issue, but when a new group was added, the tabs weren't consistent with the browser's theme. Again, it's probably an easy fix.
Saving the groups of tabs would be a great feature as well.
Experienced Firefox users are already using bookmarks in a grouping fashion by using folders and TabGroups provides a more suitable way for novices to group and organize their bookmarks.