Do you have the latest Flash plugin installed? You can get it from here:

http://fpdownload.adobe.com/get/flashplayer/current/install_flash_player_osx_intel.dmg

Yes, thank you very much. The problem was related to the Flash plugin. It had been disabled by Firefox 'for my safety' apparently. No supported versions available any longer by Adobe for ppc macs - had to download an older version from an archive site - fixed the problem. No more crashes on Firefox or Safari while on Youtube.