Try changing the editor font in preferences.txt:

Tip: don't edit preferences.txt when Processing is running - it overwrites the file on shut-down ;)

As said. I use Andale Mono, which has good distinction between 0Oo and 1lI| ( 0Oo and 1lI|) but Deja Vu Sans Mono and Bitstream Vera Sans Mono are quite good at this too.