Google Chrome is proprietary software. There's no way to back up any claim of it being "secure" because there's no way to determine what proprietary programs do. Users lack the permission to inspect the program's source code, alter the program, or distribute altered versions.
Firefox, by comparison, was never proprietary. Users were and are free to run, inspect, share, and modify Firefox (with perhaps a minor naming hurdle that apparently isn't hard to overcome as Debian demonstrated for years). In fact, this is likely a reason why TorBrowser is based on Firefox. Software freedom isn't about guaranteeing the user security, it's about addressing the inequity between users and developers inherent in non-free software. The technical advantages or disadvantages come and go and are apparently only as far as developers want to take them. People can and do learn to become software developers. And free software's technical merit can be improved by anyone willing to do the work. So we can add impressive technical features to free software. But we have no way to make proprietary software free. So the path to getting software we can evaluate against a claim of "security" and back up that claim starts and ends with software freedom.