Hi,
Paint.net compresses 8-bit bmp too much than it supposed to be. It wasn't like this in older versions (like 2 version ago i guess?)
Here's a comparison:
Adobe Photoshop 2020:
https://i.ibb.co/L00WCH0/firefox.png
Paint.NET
https://i.ibb.co/cvBKQvG/firefoxpaintnet.png