It works very well, however it does not imitate Safari's feature. In Safari you do not have to manually save a URL for Snapback: "The way Safari's SnapBack works is by silently marking a web page every time you type in a new URL, click a bookmark, or open a new window."
(In Safari you still can create a SnapBack point by going to History > Mark Page for SnapBack in the menu bar.)
SnapBack is about the only unique feature in Safari. The execution of this good idea in Firefox should be better. Main drawback, you have to save (and re-save) the URL each time you go to a website. It should remember saved SnapBack points. Even better, any time you go to a domain, it should regard the page where you entered as the SnapBack point (unless you save another URL for the site).
If this works correctly, it should help save me from opening a gazillion tabs when I'm doing research. For what I do, the tab histories can become cumbersome. The rating, so far, is for the idea. I'll be back to let you know how it works.