Resetting PRAM seems to have fixed for me (on newer machines I guess this is called NVRAM here's more about what it does). As mentioned below I upgraded my MacBook Pro (15-inch, Early 2011) to OS X Sierra yesterday and found I had blurry fonts. I tried to restart in "safe" mode as suggested but my machine would only "hang" - it would never start in safe mode. Luckily had my iPhone and could further research what to do if safe mode does not start and the suggestion was to reset PRAM. Here's what I found on how to reset PRAM if your MBP is stuck trying to reboot in safe mode (after doing this my fonts seem to be a bit better so I never went back to safe mode):
- Hold down power button to turn off Mac Book Pro.
- Let it sit for at least 10 seconds
- Read ahead then follow steps.
- Press start button to begin startup.
- After first chime press and hold down these four keys: the alt (option) key, the command key, the letter P, and the letter R key...
- Keep holding them you will here start chime.... keep holding you will hear a second start chime after the second chime releases the keys and let machine start. (In total you have heard 3 start chimes, the original and the two while you were holding these four PRAM keys)
After reboot my fonts seem to be much clearer so I never bothered to go try the booting into Safe mode.
Above I mentioned holding down the alt+command+P+R for two chime rounds, my understanding is that technically one round may do it but some old school folks said to do it twice to be sure. I don't know truth of that but that's what I did, two rounds of chimes.