[CentOS] exclude in a repo file

Hi All,

I just discovered that the remi repo has updated versions
of firefox.  This is good news, but it leads me to a
question.

Is it possible to exclude all packages except one or two
in a repo file?  The man page for yum.conf didn't give me
any hints.  I tried unsuccessfully adding this line:

exclude=!firefox*,!xulrunner*

Is there a way to set up this repo so that when I run
"yum upgrade" it will only check the firefox and
xulrunner packages from the remi repo?

c