While the default is for Firefox 3 to load tabs in the background (links from other tabs), when loading from bookmarks or external apps, Firefox will move to the foreground to load the tab.
To change this behavior:
- Enter about:config in the address bar.
- Press OK to move pass the warning
- In the filter bar enter: browser.tabs.load
- Change the settings as required to True
The three relevant entries are browser.tabs.loadBookmarksInBackground, browser.tabs.loadDivertedInBackground and browser.tabs.loadInBackground.
As mentioned, loadInBackground defaults to true; however, the other two default to False (meaning focus is switched to the tab). I set all three of these to True.