Ok, thank you, so that's why it ignores it :)
I just wanted to run my browser with a different user, because it's safer :S :(
How can I do that? I'm using wright now the "gksu" - and then run as user... so I can't make a shell script to do that, and I don't want to modify the browser's executable, and I'm not in programming, but the solution is to write an eg. C program to launch the browser, and then set SUID bit fot that C app?