Appears to use translate.google.com to provide the service, and the new one on me is that extensions apparently can't block other extensions' requests, since umatrix, ublock, etc can be set to block google, and Rethink still shows FF making attempts. So, it basically reports your browsing to Google...that makes it intentionally spyware in my book. FF's own builtin "Read Aloud" will have to suffice atm.