For FedEx, it could be a cookie issue or a blocker issue. Here's one "solved" thread for reference:

https://support.mozilla.org/questions/1374690 => If you run DuckDuckGo Privacy Essentials, use its toolbar button to make an exception for the FedEx site so it doesn't block things there

As for Firefox itself, check whether Firefox's Tracking Protection feature is blocking anything on the site. The shield icon toward the left end of the address bar usually turns a bit purplish in that case. Click the icon to learn more or make an exception. See: Enhanced Tracking Protection in Firefox for desktop.

Recent issue:

Firefox recently rolled out "Total Cookie Protection" to more users. This limits how sites can use "third party" cookies, meaning cookies from different websites than the one you see in the address bar. If FedEx shuffles you between multiple different servers (you see very different web addresses during login) then you might not get logged in on all of the related sites.

You can make exceptions to this feature when needed. More info in the following articles:

• Introducing Total Cookie Protection in Standard Mode
• Total Cookie Protection and website breakage FAQ

You can check the Web Console (Tools -> Web Developer) for messages about blocked content and about the presence of mixed or unsafe content.

• https://developer.mozilla.org/en-US/docs/Tools/Web_Console

You can check the Network Monitor to see if content is blocked or otherwise fails to load.

• https://developer.mozilla.org/en-US/docs/Tools/Network_Monitor

If necessary use "Ctrl+F5" or "Ctrl+Shift+R" (Mac: Command+Shift+R) to reload the page and bypass the cache to generate a fresh log.