[Tip] Disable New Improved JavaScript Engine (Warp) in Firefox If You Face Rendering Problems on Websites

Newer versions of Mozilla Firefox web browser (version 83.0 and later) come with improvements to the built-in JavaScript (JS/JIT) engine. The JavaScript engine in Firefox is called SpiderMonkey and Mozilla team has improved the JS engine a lot in newer versions of Firefox web browser.

Mozilla team codenamed these improvements as Warp Update internally. According to Mozilla, as the result of these updates (Warp) to SpiderMonkey, users will experience improved page load performance by up to 15%, page responsiveness by up to 12%, and reduced memory usage by up to 8%. They have replaced part of the JavaScript engine that helps to compile and display websites, improving security and maintainability of the engine at the same time.

These improvements are indeed impressive but some websites might not be compatible with this improved JavaScript engine. As a result, you might face rendering problems with certain websites. For example, if a website uses JavaScript to display forms or other components, you might not be able to fill the forms or get other components on the website due to the new improved JavaScript engine in Firefox.

If you are facing such kind of problems on some websites in newer versions of Firefox browser, you can disable or turn off the new “Warp” feature of built-in JavaScript engine in Firefox which will restore previous old JavaScript engine functionality and the websites will start loading fine.

Mozilla team has provided a hidden secret preference/flag to enable or disable the new Warp update for Firefox’s built-in JavaScript engine.

If you also want to enable or disable the new improved JavaScript engine in Firefox web browser, following steps will help you:

1. Open Mozilla Firefox and type about:config in the addressbar and press Enter. It’ll show you a warning message, click on “Accept the Risk and Continue” button. It’ll open Firefox’s hidden secret advanced configuration page i.e. about:config page.

2. Now type warp in Search filter box and look for following preference in the window:

The preference value is set to true by default, which means the new Warp feature for JavaScript engine is activated and enabled in Firefox.

3. To deactivate and disable the new improved JavaScript engine, double-click on javascript.options.warp preference and set it to false. Alternatively, you can click on the Toggle icon given next to the preference name.

That’s it. Now try to visit the website which was having rendering issues in newer versions of Firefox and now it should open fine without any problems.

PS: In future, if you decide to enable the new JavaScript engine functionality again in Firefox, open about:config page again and set the javascript.options.warp preference/flag to true.

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