If you need to turn on Javascript, type about:config into the address bar, press Enter, accept the warning, search for javascript.enabled, and make sure it is set to True.

Are you meaning Java or Javasript though? (they are totally different and unrelated)

See

• kb.mozillazine.org/About:config
• JavaScript settings and preferences for interactive web pages
• Use the Java plugin to view interactive content on websites
• How to allow Java on trusted sites

Some sites might indicate that JavaScript is not turned on mistakenly, for example, if Flash is disabled or if an ad blocker is preventing something from running. If you discover that JavaScript is enabled but you still get this message, can you provide more information about the page where it occurs (for example, you could paste the address of the page here)?