On several Google sites the fonts are rendered ugly in Firefox and Google Chrome under Windows 7 and 8:
I turned off Clear Type because most of the fonts are ugly with it.
Firefox and Chrome have the problem. If the problem is caused by a missing font the font should be substituted by another font, shouldn't it? In which case I would then need to find out what the missing font is and go download it.
In the meantime I installed all Google fonts from https://github.com/w0ng/googlefontdirectory but this didn't help. Either the Google sites with ugly fonts don't use one of these fonts or this ugly font is always ugly without smoothing or high DPI.
2nd: turn on clear type. The google sites look nice but i intentionally turned clear type of - it makes all fonts blurry. i just like crisp fonts (don't have a problem to see single pixels).
Conclusion: there are fonts that are ugly without smoothing or on low-dpi (normal monitors). a website could detect (by js) if font smoothing is enabled and substitute the font if appropriate (see link above, example at the bottom with and without font smoothing).
What obviously don't work:
Turn off HW-acceleration.
Installing all Google fonts (i think if i wouldn't have the correct font it would be substituted by a more readable one - this is what the chrome plugin does.)
Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"
Try to set the gfx.content.azure.enabled pref to false or if this didn't help disable Direct2D by setting the gfx.direct2d.disabled pref to true on the about:config page and leave hardware acceleration otherwise enabled.
I used to have the same problem with Google fonts (specially Google Spreadsheets) using Firefox. What it worked for me was the next:
Open in Firefox the document or URL that is displaying "ugly fonts". Make sure to open a Firefox window on each monitor/screen that you have available. In my case I have the display of my laptop and 2 external monitors.