Can I change the menus on the default browser?
No - as far as I can tell you can't change the menu in an application (unless that application specifically has that feature).  You could try downloading another browser like Opera, Firefox 4, or Dolphin (just to name a few).

That being said, if you still want to use the default browser, you can "long-press" on the address bar and a context menu will pop up with the option to "Share page". You can use that instead.

Hope this helps.