addTab( URL, referrerURI, charset, postData, owner, allowThirdPartyFixup )
addTab( URL, {referrerURI: ..., charset: ..., postData: ..., owner: ..., allowThirdPartyFixup: ..., relatedToCurrent: ... })
Return type: tab element
Opens a new tab that loads a page with the specified URL. The rest of the parameters are optional. The row of tabs will appear if needed.

Firefox 3.6 note

The second form of this method was added in Firefox 3.6; it allows you to specify the parameters by name, in any order. It also adds the relatedToCurrent parameter; Firefox uses this to decide whether the new tab should be inserted next to the current tab.

See Code snippets: Tabbed browser for examples.
See Preprocessing POST Data for preparing postData out of a string.