Works as expected! Gmail icons are not blurry anymore on my high DPI laptop.

Thank you very much! This was driving me that nuts, I spent a day to replace all icons for SVG variants. Now that Google decided to bring a new design, my userstyle isn't working anymore and I have no single interest in spending another day to fix it. Google should do all in SVG instead of png. Just my two cents. My screen is set to layout.css.devPixelsPerPx 1.15 because of a high res monitor. Thanks again for your extension!

Fixed:
https://github.com/scorpion421/BlurryGmailFixer

@Kirill Rakhman: Google changed the URL of the icons. I fixed this. Feel free to contact me if you fixed your version.