> Firefox automatically scales it up to 12px...
Only if you've told it to. My Firefox settings have "Minimum Font Size" set to "None". Perhaps scaling up to 12px is a default? (Edit: Also, are you sure you're not thinking of 12pt? IIRC, points are DPI-independent units and (AIUI) the traditional way of specifying font sizes in computerized typography.)
Despite my age, I still have eyes that are good enough to easily read the font sizes you're complaining about. A hugely important part of a User Agent is that it provide overrides for site design choices that the Agent's user has decided will benefit them. It's a good thing that UAs let folks like you choose a minimum-possible font size. It's an equally good thing that UAs let folks like me choose to see the choices that designer made that others criticize.
Pixels are in fact DPI-independent too, the CSS spec makes them exactly 0.75 of a point, which comes out to 96 DPI regardless of the device. Devices are then free to scale that up or down as they like: phones typically scale it down a touch because they’re held closer to the eyes.
What a bloody confusing way to define a unit of measure called a "pixel". 1px should be one device pixel, and 1pt should be 1/72 of an inch on the device!
Sheesh.
Eh, logical pixels have been a thing since at least X11, which is where we got that 96DPI thing to begin with. It certainly is confusing that they’re named the same thing though.
> ...points are DPI-independent units...
To be clear, this is a confusingly- (and perhaps incorrectly-) worded way to say "At a given point size, a particular glyph from a particular font is supposed to be the same size on the output device, regardless of its physical size or number of pixels.".