How to disable "Sign in with Google" in Firefox
Solution:
Install "uBlock Origin" extension for the Firefox (I already had it, since it's useful for other things)
Once installed, you'll see its red icon on Firefox. Click on it, and then click on settings (cogwheel on the bottom)
At the top you'll see "My Filters", click it, and in the list just add this line:
accounts.google.com/gsi/*