I installed Firefox 3.5 to "/home/user/apps/firefox/" and Flash was missing. To install it manually, do the following:
1. Download the RPM: http://fpdownload.macromedia.com/get/flashplayer/current/flash-plugin-10.0.22.87-release.i386.rpm OR select the latest rpm at http://get.adobe.com/flashplayer/
2. Open the .rpm file and copy /flash-plugin-10.0.22.87-release.i386.rpm/./usr/lib/flash-plugin/libflashplayer.so to "/home/user/apps/firefox/plugins". The file should be located at "/home/user/apps/firefox/plugins/libflashplayer.so"
3. Restart Firefox.