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.
But you have a couple of options so that you can continue to use your Zebra, Brother, and DYMO printers with Firefox.
The first option is to use version 52 ESR, which will allow you to continue to get security updates but won't attempt to autoupdate to Quantum.
The article on this page details all of the steps to take before you switch, and the link to version 52 ESR. You do have to go through some steps prior to switching based on this warning from that page:
Warning: By default, different versions of Firefox will use the same user profile data. If you install Firefox 52 ESR after using Firefox version 55 or above, you will lose your browsing history and any saved session. Other potential problems include error messages, breakage in portions of Firefox and issues with some websites but these other issues can usually be resolved with the Refresh Firefox feature.
The other option is reverting to the previous version of Firefox, 56.0.2, which also 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).
Make sure that you turn off the auto update feature so that it won't update to Quantum after you have installed it. This page details how to do this under the Automatic Updates and Security section.Per this page on Mozilla, you can just download and install over the current version to fix this with no issues.