An accessibility concern here is that pdf.js is pretty bad in terms of accessibility. While it's true that PDFs that aren't exposed as attachments already open in pdf.js, some users might be relying on the fact that if they download the PDF (to work around pdf.js poor a11y), opening it will open it in the OS default viewer like it does now. Of course, users can change the Firefox default, but this isn't trivial for novice users.
Bug 1191591 Comment 9 Edit History
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An accessibility concern here is that pdf.js is pretty bad in terms of accessibility. While it's true that PDFs that aren't exposed as attachments already open in pdf.js, some users might be relying on the fact that if they download the PDF (to work around pdf.js poor a11y), opening it will open it in the OS default viewer like it does now. Of course, users can change the Firefox default, but this isn't trivial for novice users. Having an option in the context menu to use the OS viewer might help mitigate the a11y concern. It's probably simpler/more discoverable than changing the Firefox default PDF viewer.