IIUC:
- He clicks a link in an app, it should open in the default browser, firefox in this case
- The link is opened in a tab in firefox, but the firefox window itself isn't raised/brought to the front, instead its taskbar entry flashes (like when an app requires the user's attention)
The "when I open a link a new tab switch to it immediately" is pertinent only when middle-clicking links from inside firefox itself, if this option is enabled it means: open a new tab -> Switch to it immediately.
c.f.
http://kb.mozillazine.org/About:config_entries (search for
browser. tabs. loadInBackground).