Firefox can open pdfs, and for some purposes, it's pdf viewer is advantageous. In particular, the print menu has some options not available elsewhere.
Unfortunately, it's not registered as a program that reads pdfs. I don't want it as the default pdf viewer, but I do want it on the list of programs that appear when I select "open-with".
How can I arrange that?
You could edit Firefox's Firefox.app/Contents/Info.plist and add <string>pdf</string> e.g in the first CFBundleTypeExtensions array, but that would get ripped-off at the next Firefox update.
So instead you may prefer to build a new Automator Application that will open these files in Firefox.
So head to Automator, (Applications/Automator), "New" -> "Application".
On the left panel, search for "Open Finder Items", double click or drag&drop on the right panel. In the "Open with" option, select Firefox.app.
Now is alredy time to export our App ("File" > "Export").
Then we go with the Finder to the location where we just exported the App, right-click, "Show Package Content" > "Contents" and we open Info.plist with our favorite editor (TextEdit should be fine).
Search for the string <string>*</string> below CFBundleTypeExtensions and replace it with <string>pdf</string>.
Save the document and go back to the Finder looking for a pdf file. Right-click, "Open with" > "Other..." and there select our App.
Firefox should open said pdf file and our App should appear in the menu:
This doesn't actually answer the question, as it doesn't add it to the Open With list unfortunately. It only stays in the list if it's set as Default.
Frankly it's just quicker for a one-off to drop it straight on the Dock icon or right onto any open Firefox page.
You can do this from Get Info… in the same way as selecting any already-registered app, except you select 'Other…', right at the bottom of the 'Open With:' list.
Once the file-picker dialog opens set 'Enable' at the bottom to 'All Apps' & choose Firefox.
You can then set 'Change All…' back in the Get Info pane.
Alternatively, for anyone on an OS old enough to still be able to run RCDefaultApp [Mojave or earlier] you can do it in there. SwiftDefaultApps doesn't seem to be able to do it.
The short answer is you don’t. The developer of the application registers it (via code) to open file types using Apple SDK and API calls. Then macOS let’s you choose a preferred app from all that support the types of a file.
Also, it’s slightly more complicated than just PDF or pdf extension since each file has many types that are mapped in code so the more specific type gets precedence in choosing an app over the more general types that will get consulted if the first choice doesn’t match.
Firefox is happy to pass PDF to other apps, but I’m not clever enough to find the embedded viewer and see if it can trick it to tell macOS it can open PDF outside the Firefox main app.
In the mean time, what happens if you drag that PDF into Firefox windows or the icon?
The following Ask Different answer may help:
A similar suggestion is offered here, with the additional suggestion of restarting Finder:
How can I add a new application to the Open With menu?
There is some detailed explanation of how that approach works in:
How can I modify the list of Applications under 'Open With...'?
This approach works, for instance, in specifying that BBEdit or MacVim can open .webloc files, which was your original question. Whether it works for your specific example of opening PDF files with a browser, I can't say. As @bmike points out, browsers can be more complicated.
As noted by @Kaiido above, the above approach suffers from the fact that it will be overwritten at the next app update, so you may prefer a non-Info.plist approach.
