There is currently no plan for PWA support in Firefox

tl;dr: Firefox decided to remove the Site Specific Browser (SSB) feature. This allowed to display a website inside a window with minimal UI. This feature is being removed because it was buggy, but also because it was the first step towards supporting PWA, which is not something Firefox wants do do anymore. The decision of not pursuing PWA was made following internal user researches that are currently confidentials.