From MozillaZine Knowledge Base
This preference allows for an override of the version number used when checking extension compatibility. As of Firefox 2.0b1, this preference has been replaced by extensions.checkCompatibility.
Possible values and their effects
A numeric version number (ex: 1.4.1). By default it is not defined, and extension compatibility checks uses the true application version number.
This preference used to be the source of the version number rather than an override.
First checked in
Has an effect in