well in opera, most of the menubars can be toggled on and off in View. if you turned off your File menubar, you can get that back by pressing ctrl-f11. i don't know how rh 9 does it, but usually in rh 7.3 in kde 3.X you edit your file associations to open with opera. basically, find your file associations branch in kcontrol and open up the text branch. edit all the html, shtml, and other web type documents to open with opera by adding (if opera not listed in applications to open with), and moving the opera listing to the top of the list. then click the embedding tab and and choose open in seperate viewer.
now, before you do this though (depending on if opera is listed in the applications to open with), opera says for kde you need to alter the opera command. do this in kde's menu editor by right clicking the kstart button and choosing menu editor. find opera and change the command to opera %u
this is for like when you click a link in an email, kde will pass the url directly to opera instead of looking through kde's cache for the page. in gnome i think you change the command to opera %s
after doing this you can go ahead with the first step of adding opera to your lists of applications to open with since clicking the Add button there will bring up a pop with a tree view of your kmenu.
don't know about your moz problem though.
edit: i also don't know what the steps are for making a default browser in gnome since i don't use gnome. you'll have to google or wait for that if you use gnome.