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. Normally you need to trigger this feature by pressing ' (single quote) to search link text, or / (forward slash) to search all text; however, you can have FAYT watch all keypresses to start finding.
Possible values and their effects
Start Find As You Type on any keypress. (Default)
Only start Find As You Type after pressing ' or /.
A checkbox labeled “Find automatically when typing within a web page” is located under “Edit → Preferences → Advanced → Keyboard Navigation → Find As You Type”.
First checked in
Has an effect in