Note: This feature is only available in Firefox 4. There may be currently or soon an add-on to add this functionality to Firefox 3.6 and older.
Okay, I had forgotten about this feature as it was introduced very early on around the first beta as Tab Candy by Aza Raskin. This feature later became known as Tab Groups and Panorama the viewer. Panorama allows you to view thumbnails of your tabs in their respective groups and organize these groups (rename and delete groups as well as moving tabs between groups).
As the name implies it allows you to group your tabs into well groups. This is great for people who have different projects they are working on and want to have an organized way managing the different tabs associated with each project. A simple way of looking at this is having each set of tabs in their own window, however there is only one Firefox window open and only the tabs of active group are visible. Note: App Tabs are special and will appear along the right-side of all groups.
To activate Tab Groups click the Group your Tabs button () at the end of the tab bar (this button can be moved). This will open Panorama where you will be able to start managing your tab groups. Note: what tabs are already open in the active Firefox window will be part of the first group. To add a new group double click in an empty area in Panorama and new Firefox window will open where you can start opening/adding tabs for that group. Important: opening a new Firefox window outside of Panorama will not work as this will be consider a different group. When you are done opening/adding tabs, simply click the Group your Tabs button again to return to Panorama and your new group will be visible.
The image below was me playing with this feature on one of my testing profiles. I created four groups; Firefox Extension Guru, Email Mafia (my Thunderbird site, blog and wiki) , El Guru (my personal site, blog and gallery) and a General for stuff that I access on a daily basis like Yahoo! Hotmail and Delphi Forums.
You can adjust the size of each group to make the thumbnails bigger or smaller by dragging on the bottom right corner of the group’s window. Click the X will close the group and all associated tabs (there is an undo feature if you do accidentally close the wrong group).