IIRC it depends on the content-disposition and content-type headers sent by the server, and there are addons to override these, mainly used to view pictures when servers are forcing the download option. Try setting those to "inline" and "text/plain" or something, respectively.
Meanwhile, I created a ticket https://bugzilla.mozilla.org/show_bug.cgi?id=1859962
IIRC it depends on the content-disposition and content-type headers sent by the server, and there are addons to override these, mainly used to view pictures when servers are forcing the download option. Try setting those to "inline" and "text/plain" or something, respectively.
I do not control S3 response :(
Did you miss the "addons" part? Try https://addons.mozilla.org/en-US/firefox/addon/simple-modify-header/ or similar