I love this add-on so much. If OneTab was not available for Firefox, I would not use Firefox.

Mostly, I just find it extremely convenient to be able to close out all my tabs and pick them back up again later.
My general style of browser usage revolves around having a silly number of tabs open, covering a lot of things, organized across several windows, and this add-on makes it so my computer can keep up with my ridiculous behavior.