If a website is served over HTTPS and has a valid manifest, a subtle badge will appear in its address bar. Tap this, and an "Add to Home Screen" message will appear. When the PWA is launched from the home screen, it will appear in its optimal view mode and orientation, as well as a separate entity in the app switcher, which means you don't have to worry about losing progress with it if you click an external link.
Firefox 58 also boasts a bunch of other handy features. "Add Page Shortcut" will let you put a shortcut to any URL on your home screen, while external links accessed via PWAs are opened in a custom tab for added security and a faster load time. Chrome has long spearheaded the PWA effort, but now Firefox is catching up we can expect deeper integration in upcoming releases. Android product manager Andreas Bovens says the team is "quite excited" about a number of upcoming API support products, and that Background Sync Implementation, for example, is slated for Firefox 59.