let optimalDatabaseSize = Math.min(
        memSizeBytes * DATABASE_TO_MEMORY_PERC / 100,
        diskAvailableBytes * DATABASE_TO_DISK_PERC / 100,
        DATABASE_MAX_SIZE
      );

      this._urisLimit = Math.ceil(optimalDatabaseSize / URIENTRY_AVG_SIZE);

// Max number of unique URIs to retain in history.
// Notice this is a lazy limit.  This means we will start to expire if we will
// go over it, but we won't ensure that we will stop exactly when we reach it,
// instead we will stop after the next expiration step that will bring us
// below it.
// If this preference does not exist or has a negative value, we will calculate
// a limit based on current hardware.
const PREF_MAX_URIS = "max_pages";
...
// We save the current unique URIs limit to this pref, to make it available to
// other components without having to duplicate the full logic.
const PREF_READONLY_CALCULATED_MAX_URIS = "transient_current_max_pages";
...
  _loadPrefs: function PEX__loadPrefs() {
    // Get the user's limit, if it was set.
    try {
      // We want to silently fail since getIntPref throws if it does not exist,
      // and use a default to fallback to.
      this._urisLimit = this._prefBranch.getIntPref(PREF_MAX_URIS);
    }
    catch(e) {}

    if (this._urisLimit < 0) {
      // The preference did not exist or has a negative value.
      // Calculate the number of unique places that may fit an optimal database
      // size on this hardware.  If there are more than these unique pages,
      // some will be expired.

      ...

      let optimalDatabaseSize = Math.min(
        memSizeBytes * DATABASE_TO_MEMORY_PERC / 100,
        diskAvailableBytes * DATABASE_TO_DISK_PERC / 100,
        DATABASE_MAX_SIZE
      );

      this._urisLimit = Math.ceil(optimalDatabaseSize / URIENTRY_AVG_SIZE);
    }

    // Expose the calculated limit to other components.
    this._prefBranch.setIntPref(PREF_READONLY_CALCULATED_MAX_URIS,
                                this._urisLimit);