From github: https://github.com/mozilla-mobile/android-components/issues/11745.

Firefox Android does not accept navigator.share that has files in it.

Test method:
Using https://w3c.github.io/web-share/demos/share-files.html

1. Upload an image
2. Click 'share'
3. The site reports:

Error: File sharing is not supported in this browser.

Although this is marked as closed, and the files parameter mentioned, the feature is not implemented.

https://github.com/mozilla-mobile/android-components/issues/3486

┆Issue is synchronized with this Jira Task

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:cpeterson, could you have a look please?

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I cannot find a bug about file support. According to https://searchfox.org/mozilla-central/rev/0e3779abe6ad44feb787a295fbb67bc9fe4e927a/dom/base/Navigator.cpp#1527-1531, this is TODO item, but I cannot find it.