If a website has a compatible PWA manifest the there will be an item labelled "install" in the three-dot menu of Firefox in place of usual "add to homescreen" item.
Edit: There's a few other requirements as well for the website to be considered installable as PWA, such as it must have a registered service worker so it can work offline. But regardless, if the website provides all the requirements then it can just be installed straight from the menu.