Rollback should go with re-installing the executive files, I guess. Your profile should be OK though, but remember to copy the folder "%AppData%\Mozilla\Firefox" (just write it to the "Run" window if you're not sure where it is and go back one step) to a safe place to be sure (I'm not sure what the "Local Settings\Application Data\Mozilla\Firefox" folder stores in your profile, but you can backup that, too, if you want). That's how I always did it. Just download your flavour of 188.8.131.52 from the Mozilla FTP (clicky) - I'm thinking you're using Win32, right ;D ? You should also try to update or re-install your flash plugin.
There seems to be a problem with your Windows installation or with your CD. Check them for file integrity before doing anything else. Second of all (I don't know if it's changed or not because it's been months since I've settled someone's problems with RS), I think Runescape is a Java game, not flash. So re-installing Java might help. Are you running 64-bit Windows? That might be the problem. Or have you installed IE7? If you have, try to revert back to IE6. If you haven't, try to download the Beta 2 of IE7 after you've read the notes to it (clicky). If you can bare to play the game without sounds, try to play it in Firefox. These are my advices, I hope they help.