
Mozilla Firefox currently doesn’t offer App mode for websites where in the past this was possible in the Firefox with the Prism project that was based on the Site Specific Browser (SSB) concept, the project has been discontinued a long time back. Now Mozilla is adding the ability to Firefox to run a Website as an SSB. The company now uses the familiar word App Mode over Site Specific Browser in Firefox Nightly. The highlighting point here is that Firefox now lets you install and run Websites as Apps in the Nightly version.
Google Chrome, from the beginning, allowed users to create App shortcuts and open them in their standalone window for websites, these days Chromium browsers such as new Edge, and Google Chrome support Progressive Web Apps. Whenever you visit a PWA site, these browsers will detect and display an icon in the address bar to let you know the website can be installed as an App. Now Mozilla has brought Chrome’s Website shortcuts creation feature to the Firefox browser.
Install a website as an App in the Firefox browser
1. To get started, download and install Firefox Nightly on your computer, and launch it
2. Load about: config and create a Boolean Pref named “browser.ssb.enabled"
and choose its value as “true”.
3. Restart Firefox and visit any website in the address bar, click on … icon to open the Page Action menu and select “Use this site in App mode”
Tip: You can right-click and add that icon to address bar so that, next time, you can install apps more easily by clicking on that icon.
3. The website’s shortcut will be created on the desktop and that site will open in a window with site favicon without any toolbars, browser chrome, and navigational elements when launched. You can pin the app to the taskbar to launch it quickly.

In this way, you can install as many websites as you want as apps.
To uninstall an App installed in this fashion,
Uninstalling Apps in Firefox browser
1. In the Firefox browser, click on the hamburger menu icon to open the menu,
2. Select Sites in App Mode, click on close icon at the end of a website title, that app will be uninstalled.
Recommended reading: how to uninstall PWAs from Chrome and Chromium Edge
Do note the feature being worked based on Site Specific Browser is still a prototype, Mozilla still needs to iron out plenty of bugs and issues to announce it is available for testing in Nightly. Sometimes, the app may not launch with Ublock Origin enabled for that website in the Firefox browser, this is a bug that Mozilla is aware of and is working to resolve.
Related articles:
Mozilla adds Kiosk Mode support to Firefox
Chromium Edge gets Classic Edge’s FullScreen mode
Firefox 73 lets you launch a website as a Site Specific Browser