Did you ever figure out a fix for this?
I'm having a similar issue...didn't notice if it started after EFI install or not.
All was working well with Safari then it started to hang on startup (spinning beach ball). Only way to get out was to force quit. Reinstalled Safari and worked well for a day or so, then same issue started again. I can use Onyx to clean caches and such -- it works for a while, then stops working again.
I've got Firefox too, and it behaves nicely -- no hangs at all.
I'm thinking I must have a corrupt OS X file somewhere -- so I'll probably reinstall. But since Firefox is working, I'm in no big hurry.
Thanks! And thanks everyone for all your work on the new versions and updates.