1.5 offers the "open external link in new tab" option natively, which is the option that SG has mentioned. Earlier versions required the use of the context menu that SG describes or the installation of the Tab Browser Prefs plugin.
when i do as ade.h says i get the context menu up then i select open in new tab then what happens is just bellow the Google toolbar the tab appears but that is not what i want i would like the window to appear in the same window as i am already viewing.
You can't get a new window to appear in the existing window! You are asking to do the impossible. Tabbed browsing was designed - long before Firefox and even before Opera used it - to allow multiple pages to be open without having the multiple windows that Internet Explorer currently has to use.