Facebook news feed font is displaying all blurry when it loads, a simple highlight corrects it, I have a screenshot. Seems to mostly be Facebook, although this issue has occurred elsewhere...
Reset the page zoom on pages that cause problems.
Try to disable hardware acceleration in Firefox.
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.