I was able to fix this problem by updating my Adobe Acrobat plug-in. Check if your plug-ins are up to date by going to this page.

http://www.mozilla.com/en-US/plugincheck/

Getting the plug-in to update was a whole problem in itself, as I could not get it to update at first. It took me a bunch more research and a few hours of mucking around to figure out a way to fix that.

To fix the problem of the Adobe Acrobat plug-in failing to update, within Firefox, go to Tools menu, select "Options", then in the General tab, "Manage Add-ons". Click on the Extensions tab, and then uninstall Adobe DLM. You will have to restart firefox after that.

Then go back to the plug-in check page, and then follow the links to the Adobe site and follow the instructions to update the Adobe Acrobat/Reader plug-in. (I happen to have a copy of Acrobat, and Adobe recommends not also having the stand-alone Adobe Reader installed at the same time if you have Acrobat. But for the plug-in for Firefox you install Reader and somehow it knows to make it an Acrobat plug-in.) Make sure you temporarily disable any anti-virus software that you have running first, before proceeding with the installation. The Adobe Reader X 10.0.1.434 update will reinstall the Adobe DLM extension in the process, and this time it should be successful in updating. Then close and restart firefox and go back to the plug-in check page to make sure that your Adobe Acrobat/Reader plugin is now updated.

I also gave the usps.com site extra permissions to pop up windows. I don't know if that was necessary or not. To do that, I went within Firefox to the Tools menu, selected "Options", then in the Content tab, selected "Exceptions" for block pop-up windows, which I normally have checked. Then I added usps.com.

I also changed the Enable Java Script option in the Content tab by clicking on the "Advanced" button to allow scripts to Move or resize windows, Raise or lower windows, and Disable or replace context menus. Previously I did not have the "Raise or lower windows" option checked. I don't know if that made any difference or not. Probably, just managing to update the Adobe Acrobat/Reader plug-in fixed things.

Anyway, to my relief, in this way I was finally able to get the click 'n ship printing to work. Yay!

The problem lies with how Firefox handles PDF files. Go to your Options dialog, select "Applications" and make sure that the "Adobe Acrobat Document (application/pdf)" is set to a valid application. This will return to the annoying behavior of opening all PDF files inside a browser instead of saving, but you can switch it back when you're done. Hope this helps!!