It seems I am getting blurry fonts using firefox in windows10, on a HD screen. You can see below Inbox is sharp, while the titles of the messages are blurry :
I queried window.devicePixelRatio and it is one on my screen (despite windows being set to 2.5) - so runbox is behaving correctly
I had a hunch and turned of privacy.resistFingerprinting - this is a Firefox setting that makes it harder for websites to track you, but sometimes leads to collateral damage
Result: Without resistFingerprinting, Runbox 7 is no longer blurry