On iOS (iPhone 14 Pro, iOS 16.3.1), copy a PDF URL like:
Open Firefox Focus on iOS (109.0 16317) and tap in the address bar. Choose Paste and press "go" in the virtual keyboard. Wait for the PDF map to load.
Tap the three lines "hamburger" menu in the bottom right corner and choose "Share Page With..."
Scroll through the carousel of app icons and note there is no "Books" icon. There should be because this is a PDF and "Books" is a typical use-case for a user to want to save a PDF locally to refer to offline without requiring network access.
Aside: the "... / More" option at the end of the app icons carousel shows a "Suggestions" list with "Books" which when clicked momentarily shows an alert "Creating PDF / Preparing..." (which it should not need to, the document is already a PDF), and then fails to do anything.
If you follow the same steps in Firefox for iOS (109.0 25841), and choose "Share", note that the "Books" icon is present in the carousel of app icons to choose from, and tapping it opens the PDF document directly in the Books app for offline viewing.
Firefox Focus iOS should similarly have "Books" in that carousel by default, and tapping it should similarly open the PDF directly in the Books app, without any need to create/convert anything.