Why This Add-On? Rated 5 out of 5 stars
When you have 80+ related tabs open and you need to change the subject for a while, but saving them as bookmarks is not adequate (because you need to know what site led you to a specific tab, i.e., tab history), then you will fully understand the necessity of this add-on.
It's re-purposing and expanding on an emergency backup feature built into Firefox in order to give the user fine-grained control over their browsing history. You don't want advertisers to be the only ones with that control, do you?
You're not likely to understand the purpose of this add-on unless you do a lot of research-related internet surfing that may require you to continue at a later date, It's a simple enough concept, but one that may not be appreciated by a casual user.
If you don't understand something in the first place, it's best not to rate it until you do.