Love it! Makes it really convenient to use ProtonMail on desktop without having to enter credentials & 2FA every time. Removed part: "Just one thing: the "Preferred ProtonMail domain" setting doesn't seem to have any effect. I set it to use the beta domain but it still opens "mail.protonmail.com" instead.
Edit: Developer replied with working solution. Awesome dev!

Worked perfectly at the start, notifications worked like a charm, unfortunately at some point from some reason I got a ERR on the icon and it says that my session expired and now doesn't show notifs. Tried removing and adding to no avail and assume its just my setup for Firefox that broke it. Still Pretty good.

Edit: Read this just now, yes I changed more than a few about:config settings as recommended on the PrivacyTools website but not privacy.firstparty.isolate as it was more often than not irritating. I can make an issue on github if you want.

Edit2: I did some testing of my own to see what went wrong, I have an extension called Privacy Settings which allows me to change the about:config settings at will, I made a new profile and started changing it to match my current setup till I decided to restart the profile as that might need to happen so I did and it gave me ERR,

I then set the extension to default and got the number again so I assume websites.cookieConfig.behavior which was set to block Third Party cookies was to blame.

I then made another profile and this time only changed the cookie settings in options and managed to get the same issue by blocking All Third Party Cookies in the options which means in order to make the extension work they must be enabled which I just won't do.

This is a great extension that enhances speed of protonmail web app.
Please contact protonMail team and make it official.

Amazing addon! I no longer have to keep my ProtonMail tab open.