This has been a sticking point for me too when trying to transition to FF from Chrome. My main monitor is a 4k 24-inch, and at that pixel density, the Windows HiDPI scaling isn't always enough to get the text the correct size for comfortable viewing. For text-heavy sites like HN, Reddit, and Wikipedia, being able to set a default 125% zoom in the browser is very nice, while still being able to keep the default zoom for sites like Youtube where video player size is more important than text size.

Considering the cost to implement would probably be pretty low, I think it is a fair complaint against the browser. Still, privacy concerns may supersede convenience in this particular case, and I think I might try again to switch.