Firefox recently updated to a version called Quantum that is not supported by the JS Print plugin required for Web based printing for both Zebra and Brother Printers. It has also been found to be incompatible for many sites that use DYMO printers, such as InventoryLab.
Reverting to the previous version of Firefox, 56.0.2, seems to fix this issue. We are suggesting that our customers that prefer to use Firefox revert to this version until DYMO, Zebra, and Brother have released versions of their software that are compatible with the latest version of Firefox.
You can do this by clicking on the link below ( I already selected the suggested version for you) and selecting the Mac, Windows 32 or 64 bit version needed. Next you will be asked to select the language version("en-US/" for english).
Per this page on Mozilla, you can just download and install over the current version to fix this with no issues.