This ticket tracks making the setDefaultPDFHandler Nimbus feature added by Bug 1743914 accommodate user-changed PDF handlers, by which we mean PDF handlers that are not other browsers. We're trying to avoid a "split brain" situation where a user wants Firefox as their default browser but ends up back in Edge after opening a PDF. We also want to be "respectful": we want to avoid taking the handler position from user's who've changed from the default handler (Edge), say to Adobe Reader or Nitro PDF. See also https://bugzilla.mozilla.org/show_bug.cgi?id=1743914#c5.

Technically, I expect we'll layer on another Nimbus feature, on by default, to control whether to check for a browser as the existing PDF default handler and to set Firefox as the default PDF handler only that is the case.

When enabled, this feature restricts the existing
setDefaultPDFHandler Nimbus feature to only set Firefox as the
default PDF handler when the existing handler is a known browser.

Depends on D142147

https://hg.mozilla.org/mozilla-central/rev/219c9f4056b0