There was recently an update to Firefox (v. 54) that includes some stricter security settings. In order to get your printer to communicate with your browser, you'll have to manually tell Firefox it's okay to communicate with the QZ Tray.
We want to ensure you that Check-ins is completely secure. This message is in reference to QZ Tray not having a valid certificate (they are working on an update right now). The insecurity is between the QZ Tray that is installed on your local machine and your browser. It is not between your browser and the internet so the connection is actually secure even though it looks as if it isn't.
To get Check-ins working again in Firefox can you go to the following page:
From there, you should see a page like this:
Select Advanced and then select Add Exception
On the next page, you will want to make sure that Permanently store this exception is checked and then select Confirm Security Exception.
After you do that, you might get a page with an error message on it but that is OKAY!
Go back to the Printer Check Page and refresh! You should get two green check marks next to QZ Tray and Zebra Printer. Select Send Test Print.
If you get three green check marks, you should be good to go!!