Are you reporting this from the 'sick' computer?

Are you talking about Java or JavaScript? Java I think.

On the 'sick' computer;

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control>(Mac=<Command>)<Shift> A)

On the left side of the page, select Plugins. Does Java have any warnings?

Make sure your Java (Not JavaScript) is up to date Verify Java and Find Out-of-Date Versions {web link}

Version 8 Update 121 Release date Jan 17, 2017 http://java.com/en/download/manual.jsp

To avoid confusion: http://kb.mozillazine.org/JavaScript_is_not_Java

http://kb.mozillazine.org/Java http://kb.mozillazine.org/JavaScript

Java and JavaScript are different languages.

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (e.g. Adblock Plus, NoScript, Flash Block, Ghostery) then make sure such extensions do not block content. There is security software that can interfere with loading content, so check that as well.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• Do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes