I know microsoft paint is bloat but I wouldn't go so far as to call it 'malware'. It's more like adware.
Also running any script in your browser is downloading, and running anonymous code giving it a playground to do whatever it wants. I'd trust the guys maintaining imagemagick or ffmpeg in my little corner of the web over the security team for firefox. Smaller attack surface y'know?
If they don't need to upload it why are they using a web browser at all? There are literally hundreds of programs that integrate tools to compress images.
Also running any script in your browser is downloading, and running anonymous code giving it a playground to do whatever it wants. I'd trust the guys maintaining imagemagick or ffmpeg in my little corner of the web over the security team for firefox. Smaller attack surface y'know?