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.
At this point, the only option is reverting to the previous version of Firefox, 56.0.2, which fixes this issue since it allows the use of the JS Print Extension. 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.
While it is still in Beta, you can also try using IL Print to print your labels through your DYMO or Zebra Printer. This standalone application doesn't rely on the browser or any extensions to print your labels and while it works reliably for DYMO printers, it doesn't always work for Zebra, and not at all for Brother printers at this time.
If you are on a Mac, click here for the install instructions.
If you are on Windows, click here for the install instructions.