loadOneTab( URL, referrerURI, charset, postData, loadInBackground, allowThirdPartyFixup )
loadOneTab( URL, { referrerURI: ..., charset: ..., postData: ..., inBackground: ..., 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. This method works the same as addTab except for the loadInBackground parameter which allows you to choose whether to open the new tab in foreground or background. There's also no owner parameter as the owner tab is specified automatically.
if true the tab will be loaded in the background, if false the tab will be the newly selected tab. And if null or not specified, this parameter will default to the browser.tabs.loadInBackground preference.

Firefox 3.6 note

The second form of this method was added in Firefox 3.6; it adds the relatedToCurrent parameter, and allows the parameters to be specified by name, in any order.