I had similar issues in the past. Probably it's a problem related to the Safari installation on your machine. Try with other Safari 7 installation on different machine and see if the problem persists.

Safari 7 get stuck when loading webfont (woff) from cache
On my Windows 7 x64 computer, Firefox 4 has seamlessly downgraded itself to Firefox 3.6 twice.  I haven't determined exactly when this is occurring, but the second time was when restarting after adding GreaseMonkey.  It is possible the first time was also after adding an add-on.  

Re-upgrading to 4.0 seems to work perfectly.

How can I stop this from happening again?
