Ironically, the Firefox version of this extension requests permissions to inspect all domains, not just Gmail -- indicating that the extension itself may be performing tracking.
I can't tell offhand whether the Chrome version does the same.
I can't tell offhand whether the Chrome version does the same.
As for Firefox, the reviews seem to tell otherwise, it's not been updated for over 2 years (same for chrome as well).
[1]https://support.google.com/chrome_webstore/answer/2664769?p=...