You can look at this pref on the about:config page and increase its value to see if that helps.

• accessibility.typeaheadfind.timeout = 5000

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• http://kb.mozillazine.org/about:config

In addition to the above :

Should you want to disable it altogether :

Type in the address bar about:config (press Enter)

Type in the search bar timeout

Look for the preference accessibility.typeaheadfind.enabletimeout

and set its value to false

Close and restart Firefox, in order for the change to take effect.