This addons very comfort use as "dynamic bookmarks" and I have a few good recommendation for improvement:
- folders must always collapsed default or make this setting for it. Folders good using as subgroups for left tabs groups;
- when actived tab located in folder, all tabs operation related with creating new tab (such as clone of tab in folder, new tab) must work only in current folder. -- New tabs creating in current folder;
- with close folder all inner tabs and subfolders must closing;
- add possibility unload all tabs for folders (in folder content menu);
- add possibility open all tabs in folder in new left tab group with deleting this folder;
- for economy memory your tabs must to makings (html code) during activating left tab group or during unfold folders/trees;
- real FF tabs must creating and linking only after activate your tabs;
- option unload tabs must close real FF tabs, but your tabs must markeds like unloads and stay in list.

Also all your tabs you can save in special folder of FF bookmarks and working from this place, where subdirs first level = groups for tab in left... Option for set base bookmark dir for addon can add to addon config.

If in FF enabled option about save tabs before close program, then with restoring tabs of browser, you addon just must merge self ungrouped tabs with other.

PS Addon is full replacement FF tabs, which must hide. Meanwhile I hide it through "userChrome.css".