[Affected versions]:

• Firefox Nightly 112.0a1 (Build ID: 20230312211644)

[Affected Platforms]:

• Windows 7 x64
• Windows 8.1 x64

[Prerequisites]:

• Have the latest Firefox Nightly 112.0a1 build installed and opened.
• Have the “browser.newtabpage.activity-stream.asrouter.devtoolsEnabled” pref set to true in the “about:config” page.
• Have a PDF file saved on your computer.

[Steps to reproduce]:

1. Navigate to the “about:messagepreview?json=eyJpZCI6IlNFVF9ERUZBVUxUX1BERl9IQU5ETEVSX1FBIiwidGFyZ2V0aW5nIjoidHJ1ZSIsImdyb3VwcyI6W10sInRyaWdnZXIiOnt9LCJ0ZW1wbGF0ZSI6ImluZm9iYXIiLCJjb250ZW50Ijp7InR5cGUiOiJ0YWIiLCJ0ZXh0IjoiU2V0IEZpcmVmb3ggYXMgeW91ciBkZWZhdWx0IFBERiBIYW5kbGVyIiwiYnV0dG9ucyI6W3sibGFiZWwiOiJPSyIsImFjdGlvbiI6eyJ0eXBlIjoiU0VUX0RFRkFVTFRfUERGX0hBTkRMRVIifX1dfX0=” URL.
2. Click the “OK” button from infobar.
3. Close Firefox.
4. Verify the default OS PDF handler via Start → Default Apps → Choose default apps by filetype.
5. Open the PDF file from prerequisites.
6. Open the Firefox browser.
7. Open a new tab and click the “Expand tools” button from the top right corner of the page.
8. Evaluate the “isDefaultHandler.pdf” targeting parameter on the “Targeting” section.

[Expected result]:
Step 4: The Firefox Nightly is set as a PDF handler.
Step 5: The PDF file opens through the Firefox Nightly browser.
Step 8: The “isDefaultHandler.pdf” targeting parameter is evaluated as true.

[Actual result]:
Step 4: The Firefox Nightly is not set as PDF handler.
Step 5: The PDF file is not opened through the Firefox Nightly browser.
Step 8: The “isDefaultHandler.pdf” targeting parameter is evaluated as false.

[Notes]:

• This issue is not reproducible when manually setting the Firefox Browser as PDF handler through the OS settings.
• We’ve noticed a schema validation error in the Browser Console when the infobar shows on all Windows versions. The error can be found here: link.
• Attached is a screen recording of the issue.

This is acceptable for now because our experiments are only targeting Windows 10+.
Re: the schema validation error, that was because the message link I provided did not have a trigger field. That won't be a problem with the experiment going forward.

This is expected as per comment#1