Several things to try...
From your Safari menu bar click Safari > Preferences then select the Security tab. Deselect: Enable plug-Ins. Quit then relaunch Safari. If Safari doesn't freeze, then follow the instructiions for troubleshooting third party add ons here.
If that didn't help, from the menu bar again, click Safari > Preferences then select the Privacy tab.
Click Details then click: Remove All Website Data
If that didn't help, might be corrupted Safari preferences (.plist)
Go to ~/Library/Preferences/com.apple.Safari.plist
Move the com.apple.Safari.plist file from the Preferences folder to the Desktop.
Relaunch Safari. If that helped, move the .plist file to the Trash. If not, move it back to the Preferences folder.
If that didn't help, quit Safari then open the Finder. From the Finder menu bar click Go > Go to Folder
Type this exactly as you see it here: ~/Library/Caches/com.apple.Safari/Cache.db
Click Go then move the Cache.db file to the Trash.
Qui then relaunch Safari to test.
If nothing above has helped, try Safari in another user account. Same problems there is an indication that the startup disk may need repairing. For that you need Mountain Lion Recovery.
Aside from the obvious problems, when Apple got rid of the Activity window AND rss feeds, I really got peeved! Bring those back, Apple!!!
This is a user to user forum. In case you aren't aware, you are not communicating directly with Apple.
However, you can submit feedback > Apple - Safari - Feedback