i found the solution for this and i post it because your fonts are ugly and trully harm people eyes.

instead of enabling the antialiasing inside fonts settings, keep it disabled.

download a good font from the internet, personally i prefer tr_tahoma which is clear and a little condensed.

install the font and in font settings use only that font in bold.

For Firefox:

in firefox go to -> about:config

search for "font.name."
find the lines:

font.name.monospace.<your country>
set it to
TR Tahoma Bold

font.name.sans-serif.<your country>
set it to TR Tahoma

font.name.serif.<your country>
set it to TR Tahoma Bold

then only search for font
find and change :

browser.display.use_document_fonts
set it to 0
reader.font_type
set it to TR Tahoma

lastly add the non existing any more:

gfx.font_rendering.cleartype_params.rendering_mode
and set it to 1.

Done. Firefox is silky smooth again.
Thats all. Now your os and your browser are rendering the fonts just fine.
Of course you can do that with any font you like but where is bold keep it bold.
Bold is the key
If KDE ever fixes that antialiasing problem until the year 2020 it would be a miracle.
Cheers.