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. After a set amount of time with no further keyboard input, Find As You Type can be deactivated. This preference determines whether or not that happens.
Possible values and their effects
Allow FAYT to be deactivated after a set amount of time with no more keyboard input. (Default)
Once activated, FAYT will not deactivate until the user presses [Esc].
A checkbox labeled “Clear the current search after a few seconds of inactivity” is located under “Edit → Preferences → Advanced → Keyboard Navigation → Find As You Type”.
First checked in
Has an effect in