Show the issue and or provide steps to replicate. Hard to help with something we cannot see. What OS? What version of Firefox?

What happens when you try to print? Does Firefox display an error message, or just seem to load the preview endlessly, or have print errors, etc.?

(This is just the reason I am not able to investigate this myself)

I don't use the FedEx site very often, but I noticed it would not sign me in correctly -- it just immediately threw me back out to the home page. I had to clear cookies and site data before I could stay logged in. In case you haven't already tried that, here's how:

While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm. Then try reloading the page and signing in again.

For me, though, it seems to now want me to create a new account number for various features. I'm going to pause here since I don't have any immediate shipping needs.