Nightly builds of Firefox have introduced the ability to install websites as applications

In nightly builds Firefox, on which the release of Firefox 75 will be based, has added the ability to install and open websites as applications (Apps), allowing you to work with the site like a regular desktop program. To enable this in about:config, you need to add the setting 'browser.ssb.enabled=true', after which the option 'Install Website as App' will appear in the context menu of page actions (the three dots in the address bar), allowing you to place a shortcut for opening the current site separately on your desktop or in the applications menu.

Development continues to the development of the concept of 'Site Specific Browser' (SSB), which implies opening a site in a separate window without menus, an address bar, and other elements of the browser interface. Only links to pages of the active site open in the current window, while clicking on external links leads to the creation of a separate window with a regular browser.

Source: opennet.ru