The NoScript Firefox extension provides extra protection for Firefox, Seamonkey and other mozilla-based browsers: this free, open source add-on allows JavaScript, Java, Flash and other plugins to be executed only by trusted web sites of your choice (e.g. your online bank).
NoScript also provides the most powerful anti-XSS and anti-Clickjacking protection ever available in a browser.
NoScript's unique whitelist based pre-emptive script blocking approach prevents exploitation of security vulnerabilities (known, such as Meltdown or Spectre, and even not known yet!) with no loss of functionality...
You can enable JavaScript, Java and plugin execution for sites you trust with a simple left-click on the NoScript status bar icon (look at the picture), or using the contextual menu, for easier operation in popup statusbar-less windows.