Although the requirements of the USPS - The United States Postal Service (U.S. Postal Service) Click-N-Ship service require Microsoft Internet Explorer 5.0 or higher, or Netscape Navigator 6.0 or higher, you can use the online service with Firefox on a Macintosh, by saving your printed label to your computer and then renaming it from:
com.usps.cns.web.pdf.LabelGenerationServlet
to
com.usps.cns.web.pdf
and then printing it with Adobe Acrobat or Preview.
Update:
Please add the ability to CHANGE a Click-N-Ship address. The current process to request a credit. then re-enter the whole order is painful.