After installing and testing KDE's Dolphin for a while the font rendering (or maybe anti-aliasing?) in non-GTK (e.g. Chrome and Thunderbird) programs looks strange. Playing around with QT(4) and KDE settings didn't help.
Example of right render in Nautilus and the wrong in Thunderbird:
~/.fontconfig/*
file and getting rid of it solved the problem. But I am not sure --- so a comment and not an answer.