Hi,
There was a previous post dealing with this. See:
http://www.fedoraforum.org/forum/showthread.php?t=26632
The problem is with the write permisions for the search engine folder located at:
/usr/lib/firefox-1.0.4/searchplugins
A quick way, but a way that makes that folder writable by all users, would be to change the permissions as follows:
su -
cd /usr/lib/firefox-1.0.4
chmod -R go+w searchplugins
exit
You sould now be able to add search engines.
If you wanted to secure it after you have added you search engines you could change the permissions back with:
su -
cd /usr/lib/firefox-1.0.4
chmod -R go-w searchplugins
exit