Finally, found best gmail checker extension (tested on 55.0.2 64 bit)

ATM, this is the best WebExtensions Gmail checker/button. The built-in popup isn't annoying and if you really want just a button and notifier to Gmail you can just choose so.

By default this is broken in Fx57 though, because Tracking Protection enabled seems to inhibit the addon from logging in. Not sure if this is a Firefox or addon bug.

Checking it now

A lot of reviews are probably from competitors since this is the best one I've used and I've used them all.

Love you
More power

After using another much more rudimentary add on, this well written and continually updated add on is a pleasure to use.
Don't feel the need to go into specifics as it's all in the description, suffice it to say if you use Firefox and Gmail this is a must have add on.

I have used the Gmail Checker before, but now it is broken by changed in authentication process. I have installed this app and I really comfortable with it - i find the opening pop-up windows where you can manage your accounts and mails is really super!

The best extension I have ever used to manage Gmail accounts.
Five stars for sure!

This was the ultimate mail checker extension on Chrome and now that I've switched to Firefox, I'm very pleased to see that it's available here, too. However it doesn't work on Firefox 53 it appears. After setting up everything as usual, the icon shows an x in the upper corner and is greyed out. I can't even open the settings anymore. I only get an endless loading icon…

I have just switched over from Chrome to Firefox, and to my utter delight, I found out that Checker Plus is available for Firefox as well!

This extension is a must-have extension for anyone who uses Gmail to any degree. THe extension is intuitive, the layout/UI is efficient, and it is very hard to use a browser without it once you get used to it.

Of equally great import - the developed, Jason, is a thorough professional. If you have any trouble with his extension, be sure to email him or to leave your email address when you remove the extension from your browser. He WILL get back to you promptly, and he WILL fix your problem promptly.

Very Good! It fllow me from Chrome to Firefox. I it the best in same type plugins.

Very useful

My favourite mail notification extension from google chrome! So glad it's here on firefox now too.

Great addon!!