From MozillaZine Knowledge Base
Though the functionality is not present, Mozilla is designed to eventually accommodate alternate devices for enhanced accessibility via an “access proxy.” At startup, if this preference (or accessibility.usetexttospeech) is present and populated, additional event handlers are attached to windows for the proxy.
Possible values and their effects
As this functionality is not present yet, the only valid value is an empty string. Populating this preference may have unpredictable and detrimental results.
First checked in
Has an effect in