Having a searchable search history feels kinda meta, but this basically solves the problem of "I know I found this webpage a while back but I can't find it again". Ideally this sort of thing would be handled by the browser's history feature and stored locally, but the browser can't (or at least doesn't) correlate between "you searched for this term" and "you clicked this result". I also find that the browser's history search leaves something to be desired, and let's not increase Firefox's memory footprint even more by making it better!
(Browser history is also stored online for most browsers these days, and especially if you use Chrome then Google already have your history anyway).