I use Firefox's on-page search tool all the time. One thing that's always bugged me about it, however, is that it can only remember one word or phrase at a time. If you're working on a research paper, or switching between the same few words every time you use it, I recommend downloading FindList. This new, experimental extension gives your search box its own drop down history list.
With it installed you can have up to 15 of your latest searches saved for later use and quick selection. Every time you do an on-page search it simply saves that word or phrase in its history even after you start searching for something else. Neat.