From MozillaZine Knowledge Base
Find As You Type (or Type Ahead Find as it was originally called) allows you to start finding text on the page immediately by simply typing the text you’re trying to find. In Firefox, this functionality is closely tied to the Find Bar. Firefox 2.0 includes some changes to the Find Bar; for example, an Emacs-like case-sensitivity mode is now available. This preference determines how case-sensitivity is handled.
Possible values and their effects
All searches should be case-insensitive. (Default)
All searches should be case-sensitive.
Auto-determine whether the search should be case sensitive (if the search string contains upper-case characters but isn’t all-caps, consider the search case-sensitive)
First checked in
Has an effect in