This takes a few steps:

(1) Type or paste about:config into the address bar and press Enter or tap the Go arrow.

(2) Type javas into the search box on that page and press Enter.

(3) Under javascript.enabled tap the Toggle button to switch it from true to false.

If you envision switching this frequently, I suggest bookmarking about:config.

• Type about:config in URL and press Enter
• Search for script
• Scroll down to javascript.enabled and tap Toggle to switch it to false.

The "Enable/Disable Javascript" checkbox in Preferences should be brought back. Put it under Advanced if you think it needs to be made clear that it's somewhat hazardous and can break stuff. Going to about:config for it is way more cumbersome.