Bug 1703578 added a general Windows UserChoice implementation, and used it for "https", "http", ".html", ".htm". This ticket tracks generalizing the interface just a little bit further, to additional file extensions, in particular allowing to set Firefox as the default PDF handler.

This is a straight-forward extension of the existing UserChoice
mechanism to arbitrary file extensions. This is not a broad
generalization but we don't foresee many extensions beyond the
".pdf" file extension at this time, so let's not over-rotate towards
generality.

This is Windows 10+ only, since we really only care about PDF handling
defaults where Edgium is the OS default.

The model, _isDefaultBrowser, is extremely defensive. I don't think
that level of defensive programming is justified here. If
ShellService.jsm doesn't exist, or doesn't pass through to the
underlying XPCOM shell service, or fails on non-Windows operating
systems -- that's all fine.

Depends on D132658

Depends on D132659

Could you please not taking over .pdf handler from Acrobat Reader? (It is fine to take over from Edge/Edgium.)

(In reply to Masatoshi Kimura [:emk] from comment #4)

Could you please not taking over .pdf handler from Acrobat Reader? (It is fine to take over from Edge/Edgium.)

:emk: request noted. This particular ticket is about enabling the simplest possible experimentation, just changing what set to default means to include PDF handler. If it's worth it, we'll want to do something where we give users more options around PDF handling. I've been using the term "respectful" to mean that we don't trample non-default (i.e., non MSEdgePDF) user choices.

Comment on attachment 9254456 [details]
1743914-data-review-1.md

DATA COLLECTION REVIEW RESPONSE:

Is there or will there be documentation that describes the schema for the ultimate data set available publicly, complete and accurate?

Yes.

Is there a control mechanism that allows the user to turn the data collection on and off?

Yes. This collection is Telemetry so can be controlled through Firefox's Preferences.

If the request is for permanent data collection, is there someone who will monitor the data over time?

No. This collection will expire in six months.

Using the category system of data types on the Mozilla wiki, what collection type of data do the requested measurements fall under?

Category 1, Technical.

Is the data collection request for default-on or default-off?

Default on for all channels.

Does the instrumentation include the addition of any new identifiers?

No.

Is the data collection covered by the existing Firefox privacy notice?

Yes.

Does the data collection use a third-party collection tool?

No.

Result: datareview+

Backed out for bc failures on browser_setDefaultPDFHandler.js

Backout link: https://hg.mozilla.org/integration/autoland/rev/2ee98d9d6ecbf92be513f95c6b699554edf84053
Log link: https://treeherder.mozilla.org/logviewer?job_id=360870589&repo=autoland&lineNumber=2131

This was backed out 'cuz the test API for Nimbus changed overnight. Relanded after rebasing and updating the test API.

Backed out for causing failures at browser_setDefaultPDFHandler.js.

Backout link: https://hg.mozilla.org/integration/autoland/rev/da3376c0199f8deaaa276f3377c2aa41f75313fb

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=DDT8DxHuSXa1zOrNr_h83g.0&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&revision=01e19b8ec15b7cf88af181b43b1a7e2003ca52af

Failure log: https://treeherder.mozilla.org/logviewer?job_id=360885187&repo=autoland&lineNumber=4970

Try push with a green Win7 bc test: https://treeherder.mozilla.org/jobs?repo=try&revision=31a44439b489028712db5777699d4a8fe8295220. Relanding.

https://hg.mozilla.org/mozilla-central/rev/05b9d17d7a0a
https://hg.mozilla.org/mozilla-central/rev/39ca047920ba
https://hg.mozilla.org/mozilla-central/rev/ce23c0066bac

QA has performed a check on the latest version of Firefox Nightly 97.0a1 (2021-12-14) and can confirm that the browser can be set as the default pdf handler. Firefox is registered as the default pdf handler (checked with registry editor and as well with the Windows default apps settings).
This was tested on Windows 10.