When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

1. Clear Firefox's Cache

Edit > Preferences > Advanced
On the Network mini-tab > Offline Storage : "Clear Now"

2. If needed, delete the site's cookies here

While viewing a page on Youtube, right-click and choose View Page Info > Security > "View Cookies"

Then try reloading the page. Does that help at all?

You can enable and disable JavaScript in a variety of ways.

(1) Edit > Preferences > Content

There is a checkbox to globally turn JavaScript on and off. Make sure this is turned on.

(2) The NoScript add-on (do you have this?)

This extension lets you turn JavaScript on and off on a host-by-host basis. Use the "S" logo on the Add-ons bar to make those choices. Note: Youtube uses multiple domains to serve content. You probably need to enable more than one.

(3) Other add-ons

Individual add-ons may block some or all features of JavaScript for reasons related to privacy or blocking certain kinds of annoying content. You can review your add-ons under Tools > Add-ons. Pay particular attention to Extensions.

(4) External security software

I'm not familiar with Linux security software, but on Windows, some security suites can strip or block scripts before they reach Firefox, or require you to update a whitelist of sites (e.g., parental control software).

(5) Proxy services

Some services might filter scripts or other "active content."

did not help clearing cache or cookies. mozilla did say that they disabled some plugins due to hackers this year and that we should update. I did update and still nothing. Even tried the chromium browser.

Even tried the chromium browser.
Did that work?

Is this problem limited to sites that display videos, or is JavaScript not working on other sites as well?

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

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

java works just bland window for videos