How to Enable and Test Upcoming Nova UI in Mozilla Firefox
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1. Type about:config in the address bar 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 nova inside the preference search box and you’ll see following preferences:

browser.nova.enabled

browser.newtabpage.activity-stream.nova.enabled

browser.urlbar.nova.featureGate

browser.urlbar.quicksuggest.ampTopPickUseNovaIconSize

UPDATE: The previous browser.design-tokens.nova preference to enable Nova UI has been replaced by browser.nova.enabled preference in the latest Beta build.

The preferences values are set to false by default, which means the new Nova UI is not activated and enabled in your Firefox Beta version.

To activate and enable the upcoming Nova UI in your Firefox browser, double-click on browser.nova.enabled preference and set it to true. Alternatively, you can click on the Toggle icon given next to the preference name.