Several things to try...
From your Safari menu bar click Safari > Empty Cache. See if Safari freezes. If it does, back to the menu bar click Safari > Reset Safari. Select the top 5 boxes, click Reset. If that didn't help...
Might be a corrupted preferences. Quit Safari.
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 it doesn't freeze, move that .plist file to the Trash, otherwise move it back to the Preferences folder.
If it wasn't a preference issue, go to Safari > Preferences then select the Security tab.
Deselect: Enable plug ins. Quit then relaunch Safari to test. if it doesn't freeze, then you need to troubleshoot Safari add ons > Safari: Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues
If it's not a plugin issue, try Safari in another user account. If it crashes there, that can be an indication that the startup disk needs repairing.
You need to use Lion Recovery to do that.