The interpreter to handle JavaScript is built in.

To turn the JavaScript interpreter on and off for all pages:

Firefox > Preferences > Content tab

If you already have JavaScript enabled on this tab but some sites tell you that JavaScript is not enabled, this might be the result of an add-on, a script filter, or many other possibilities.

You didn't provide a lot of information in your post. Can you indicate where you have this problem, when it started, and whether you recall making any changes in Firefox or on your system before this happened?

You can start Firefox in Safe 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