My Firefox had a Gmail session active, then I was needed to make a fresh install of Opera browser. After the installation of Opera finished, I pretended to log in into my Gmail, I was expecting it was necessary to me to set my username and password and then my phone verification, but no, my session automatically opened.
From previos experiences, when I pretended to start session into Gmail from Chrome (or any new browser) it required me phone verification and username with password.
To me that is not a problem, but why did it happen?
Best Answer
I'm speculating here, but a couple of possibilities come to mind:
you already had an active session with your Google account and signed in from the same IP address; it seems logical to assume it was you
Did you copy your Firefox configuration to Opera? Did that include cookies? User agent? Google may have believed that you were using the same browser, especially since it was using the same IP address for your active session
perhaps neither of these things are absolutely true, but instead Google only offers the extra security if it doesn't have a high degree of certainty who you are. I'm sure Google has a number of things they look at to make that determination