In the documentation:
Buttons Save or Fork are always present in the UI. First one appears if no fiddle was loaded, the latter is used to create a new fiddle from the existing one.
I ONLY see SAVE when the fiddle is brand new, then RUN/update/fork. In Fx4 and Safari 5 on MAC (and Fx 4 on pc)

UPDATE: New BASE functionality does exactly what I wanted.

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When you log into JsFiddle, you’ll get a Set as Base button, which will make the revision you’re working on the base version – think of it as an alias for john/7hd62/12/ -> john/7hd62/.