Nothing since 0.9.3 will open on my Mac

Can anybody help me?

I've been using Calibre on my MacBook Pro running OS 10.5.8 for a while, updating frequently. Then starting with 0.9.4 and later I get a "Quit Unexpectedly" message on launch and the program won't start. Fortunately I can back-install 0.9.3 and all is well.

Any idea what could be causing my "failure to launch" with newer versions of Calibre? I love the app and hate to think I can't keep it up to date.

Thanks for any ideas on this.