1. A method for session-based matching of mutable browser identifiers, the method comprising: storing information in memory regarding: a plurality of mutable browser identifiers, each mutable browser identifier having been generated to be unique to a different associated website, wherein a corresponding mutable browser identifier has been provided to a local cache, and wherein a new mutable browser identifier unique to the associated website is generated upon request and provided to the local cache, and one or more maps, wherein each map associates a different immutable browser identifier with one or more of the mutable browser identifiers, the immutable browser identifier unique to a different browser in use at an associated computing device; receiving information from a computing device that has used a browser during a session to download a website, wherein the downloaded website includes a reference to a mutable browser identifier specific to the downloaded website; identifying that the referenced mutable browser identifier corresponds to one of the stored mutable browser identifiers based on a comparison of session information for the session in the received information to session information associated with the corresponding stored mutable browser identifier; and mapping the referenced mutable browser identifier to the corresponding stored mutable browser identifier based on a match between the session information for the session and the session information associated with the corresponding stored mutable browser identifier.