If I open a web browser (either firefox or chrome) while I have PS open, photoshop freezes. Just about every time. And it'll stay that way until I force quit and re-open.
This does not happen if I first minimize photoshop and then click my desktop so that the photoshop toolbar at the top of the screen gets replaced by the Mac toolbar, and then open a web browser. If, however, I open a web browser while I have the photoshop window up, it freezes. Reliably.
It's a silly problem, but very annoying, and occasionally devastating if I haven't saved my work.