That's built into Firefox 2 and Firefox 3. Just find the option and check it.

Tools, options, advanced, general, "Search for text when I start typing".

I like it, use it constantly, but wish it worked slightly differently: When you press ctrl-F, a fully featured searchbar appears. When you Just Start Typing, a similar searchbar appears but without the full features and without a FIND NEXT button. You have to guess what the keyboard shortcut is for FIND NEXT in order to use the Just Start Typing feature. I wish it simply made the full searchbar appear when you Just Start Typing.