I found it's my extension[1]. That's what's interpreting the lack of Content-Type as indication that Firefox is going to force a download (it actually doesn't)

I found it's my extension[1]. That's what's interpreting the lack of Content-Type as indication that Firefox is going to force a download (it actually doesn't) and showing me its own download dialog. [1] https://github.com/Rob--W/open-in-browser https://github.com/Rob--W/open-in-browser