I just installed the new Safari 4 Public Beta after reading about it on apple.com. I, like many users have run into the issue where both Safari and Mail.app will quit momentarily after launching it.

To fix Safari:
Uninstall Glims plugin. This can be done by removing Glims from the following paths:

/Library/InputManagers/Glims /Library/Application Support/Glims

Source: MacHangOut

To fix Mail.app:
Even though I didn’t have it enabled, growlMail was still breaking Mail.app. This can be fixed by removing GrowlMail from the following paths:

/Library/Mail/Bundles/ ~/Library/Mail/Bundles/

On a side note, I also removed Google Gears for Safari as I though this may be causing the issue aswell. And just like one of the earlier Safari 4 Developer Betas, TinyMce also fails to work correctly.

UPDATE: Upgraded to the latest Webkit Nightly. TinyMCE is working fine. ;)