Some sites use different subdomains for their checkout pages. Make sure Tracking Protection is disabled for all the sites involved in the checkout. You can also try reloading the webpage while bypassing the cache using one of the following steps:

• Hold down the Shift key and click the Reload button in the navigation toolbar.

OR

• Press Ctrl + F5 or Ctrl + Shift + R (Windows and Linux)
• Press Command + Shift + R (Mac)

Let us know if this solves the issues you are having.

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox shows the shield icon at the left end of the location/address bar in case Content Blocking is blocking content.

• click the shield icon for more details and possibly disable the protection

You can check the Web Console about what content is blocked

• "3-bar" menu button or Tools -> Web Developer
• https://support.mozilla.org/en-US/kb/content-blocking

You can check the Web Console (Tools -> Web Developer) for security related messages.

• https://developer.mozilla.org/en-US/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/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.