If you backport only firefox-3.0 and xulrunner-1.9 from hardy to gutsy, that's expected.
You also need to backport cairo (1.5.8) and fontconfig as they contain the sub-pixel LCD filter.
Without those two, xulrunner-1.9 will switch to internal cairo at build time, which is not patched and then ff3 will have no sharp font as you described.
If you backport only firefox-3.0 and xulrunner-1.9 from hardy to gutsy, that's expected.
You also need to backport cairo (1.5.8) and fontconfig as they contain the sub-pixel LCD filter.
Without those two, xulrunner-1.9 will switch to internal cairo at build time, which is not patched and then ff3 will have no sharp font as you described.