[Affected versions]:
Nightly 81.0b5

[Affected platforms]:
Platforms: Ubuntu, Mac OS

[Preconditions]
Reach about:config and set the following pref: browser.download.viewableInternally.enabledTypes = SVG,XML,PDF,WEBP,AVIF
image.avif.enabled = true

[Steps to reproduce]:

1. Launch the Firefox browser and reach https://mime.ty.ax/
2. From the preBuilt section select the AVIF link

Expected Result:
The Open in Firefox from the Open files prompt should be selected by default.

Actual Results:
The Save File option is selected by default in the open files prompt when trying to open AVIF file types.

I think the severity for this issue should be an S3/S4 since the option is still there and the user can just select to Open the file in Firefox.

In macOs, from prebuilt section, under content-disposition: attachment, default content-type and extension, only PDF link shows "open in firefox"selected by default, the remaining files (XML, SVG, WebP and AVIF won't)

Then, webp Link won't show "open in firefox"selected by default either.

Under default content-type, no extension, all files show "open in firefox"selected by default except for AVIF Link

Attaching prompt messages to demonstrate.

This is a complicated situation, since the default can depend on what other applications are available in the OS. I don't think we are going to force the default for the internally-openable files, but I'll leave this open in case we want to consider it.

(In reply to Adam Gashlin (he/him) [:agashlin] from comment #6)

This is a complicated situation, since the default can depend on what other applications are available in the OS. I don't think we are going to force the default for the internally-openable files, but I'll leave this open in case we want to consider it.

Isn't this related to whether we determine the right mime type / file extension? We've had to fix brokenness here on Windows (cf. bug 1571247 ), I wouldn't be surprised if we need to do the same for other filetypes on other OSes...