Safari doesn't remember tabs in Mountain Lion
Undo quitting of all applications
Apple has slightly changed the wording but the location for this setting is still the same as in Lion:
System Preferences → General → Close windows when quitting an application
Undo quitting for a specific application
This works only with Close windows when quitting an application disabled at the same time.
Type the following command in the Terminal. Then restart the specific application.
In general the syntax is:
defaults write com.&quot;producer&quot;.&quot;program-name&quot; ApplePersistenceIgnoreState YES

Thus, in your case (Safari.app) it is:
defaults write com.apple.Safari ApplePersistenceIgnoreState YES

You can undo this by using the parameter NO instead.
If you prefer a graphical interface, you can use TinkerTool. But the current version 4.9 only supports the toggling on a per app basis in Lion. I'll update this answer when TinkerTool adds this feature for Mountain Lion as well.