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
CFBundleTypeExtensions and replace it with
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: