Version 4 Public Beta (5528.16, r41521)
In the most recent build the fade commands toss an error like:
TypeError: Result of expression '(o.fx.step[this.prop]||o.fx.step._default)'  is not a function.
Functionality works fine in Firefox, and, I believe, in Safari nightlys as recent as two weeks ago.
I can confirm this bug is affecting the latest nightly (r41521). More info can be found via this link.
I found that page because I was having the same problem. jQuery (v1.3.2) effects seem to be broken, fade in particular, but also the Superfish dropdown menu plugin, but that may just be a side effect.
This is a duplicate of bug 24447, which is fixed in the r41545 nightly.
*** This bug has been marked as a duplicate of 24447 ***