Choose an application to open the com.emclient.mailclient link

Installing on Mac using High Sierra. All goes well and I try to set up a gmail account. I disable encryption and everything except mail. I Click Finish button and Google authentication opens in Firefox window. I login and agree to let the account access my Google services. I get this:
Screen Shot 2021-08-25 at 10.15.59
Without me doing anything, this disappears after a few seconds and goes to the Google search page.

See reply for what happens next. (It will not allow two screenshots on this post.)

Meanwhile, emclient is sitting there like this:

10 minutes later it is still doing this. How long should I wait?

Could be there is a later version of eM Client where that issue has been fixed, or could be you need to allow your Google account via a different browser, or could be that your OS version needs updating,

First check the eM Client version history page that you the latest Mac ver. eM Client] [Release History]

Failing that try setting “Safari” or “Chrome” as your default browser and see if that makes a difference.

Lastly there should be OS upgrades available on your Mac. eM Client recent updates was for “Big Sur” OS. Can you try upgrading to a more recent OS if even “Safari” or “Chrome” allow doesn’t work. Looks like there has been Mojave, Catalina & Big Sur upgrades available since your High Sierra OS.

If you do upgrade your Mac OS, make a “Time Machine” Backup first incase you need to rollback.