Update (Aug 15, 2017): We have confirmed that Check-In printing works successfully with the latest version of Firefox ESR, 52.3.
You can find detailed instructions about how to set up your printers on this Help Article.
Update (July 6, 2017): We have determined how to successfully print Check-In labels using Firefox ESR 52.2.0.
If your machines are already updated to this version or you are interested in updating, you can find detailed instructions about how to set up your printers on this Help Article. You can still remain on Firefox ESR 45.9.0 if you prefer, and instructions for restoring to previous versions and stopping automatic updates can also be found on that Help Article.
On Friday, June 23, 2017, we became aware of an issue with Check-In printing when using the latest version of Firefox ESR (version 52.0 and above).
If you are using this version of Firefox, we recommend going back to Firefox ESR version 45.9.0. You can download that version of Firefox here, for Windows, or here, for a Mac.
If you run Check-In using Firefox ESR version 52, your labels will not use the printer settings you entered and will most likely not be usable.
For updates on this issue, follow us on twitter at @CCBTech.