it looks modern with good features, but it's not what it seams to be.
Auto login always fails.
Manually account is "forgotten" several times, very often asks for login and permission - again and again.
it looks like it does not actualize very often, i have to do it manually.
The configuration is not actualized for FIREFOX users.
Help and config only for Chrome.
simply doesn't do for what it's builded for, at least NOT in Firefox, and it's laggy very often, sometimes fast.
I cannot recommend it anymore after using it 2 Weeks.

Many thanks to author. I moved on from GMailChecker https://addons.mozilla.org/en-US/firefox/addon/gmail-checker/ and want to say that CheckerPlus is a gorgeous and stunning add-on.

Esta misma extensión en Chrome es muy buena, de las mejores, pero en Firefox se notan las carencias del programador en este entorno porque es una extensión muy pobre.