Thanks for ur answer, it had put me on the right track to solve my problem.
So, here is the modified function:
function TabSelecting(sender, eventArgs)
var tab = eventArgs.get_tab();
var navigateUrl = tab.get_navigateUrl();
The navigateUrl gets the link which is supposed to be loaded when the user clicks a tab.
I get this link and then, call another function "parent.LoadURLOnglet(navigateUrl);" ( which loads this link separately in another frame - that's how my system works)
Finally, I cancel the eventArgs.
This seems the easiest way to hide the "selected" tab effect.
Another way can be to set the CSS hover tab as the normal link tab.
Hope I talked sense here, please feel free to share your part.
Cheers from Mauritius,