It may be possible that after you make a clean install of the Firefox 3, the flash plugin will not work even after the browser notifies about the successful install of the flash plugin. In this case follow these simple steps.
1) Extract the contents of the Firefox 3 package into a folder, say firefox, in /usr/lib folder. Then navigate to /usr/lib/firefox/plugins. If you don`t find a file named libflashplayer.so then you`re in trouble, but don`t panic.
2) Goto http://www.adobe.com/go/getflashplayer. And select the .rpm version of the plugin. It is approx 2.9 MB in size and should download easily. Install the package, and then extract the libflashplayer.so file from the package (yes you can extract files from an rpm package too, just right click on the rpm package and select extract) to /usr/lib/firefox/plugins folder on your computer. Now start your firefox3. The browser will report the presence of the flash plugin.
3) NO-SOUND Issues: If you find that you cant hear the sound in the flash content which is played on various sites, then your borwser most probably doesn`t have libflashsupport. Just, start a terminal and install it by issuing this command as root, yum - y install libflashsupport. Restart firefox. The sound problem will now be fixed.
Update: You could also try installing the swfdec-mozilla plugin instead of the adobe-flash plugin, via yum. I am currently running it on my fox, and there doesn`t seem to be any problem at all.
Still, finally if all the above don`t seem to work, or may be you can`t turn things around then do the following:
1) Download firefox 18.104.22.168 again and extract the contents to your home folder in a firefox folder and install the flash plugin for it.
2) Delete this firefox folder and Don`t delete the .mozilla and .firefox directories from your home dir.
3) Now download the firefox 3 package and install it at the same firefox folder.