Because of Famrir's genetic makeup, the blending of symbiote and host was being rejected. When Ra discovered Earth, he quickly took a young human male as his host. It is unknown what, if anything, Ra intended for the fate of Famrir but he was soon spirited away by a Tok'ra operative within Ra's ranks.
The Tok'ra managed to get Famrir back to his people and the Asgard welcomed him back into their fold, using their advanced technologies to heal his body; what they did not manage to heal was his mind. The memories he had gained since blending with Ra's symbiote drove him mad and so he started upon a campaign to wipe out every last Goa'uld in existence. Due to his madness, he cared little for any innocents who got in his way and simply lead as many reckless assaults on the System Lords as he could. (RPG: "System Lord Plot Hooks")
Daniel Jackson later learned of Ra coming to Earth in the body of this dying alien (though at the time he did not know of the Asgard), and of Ra transferring into the human male, through hieroglyphics on the planet Abydos. (Stargate)
Behind the scenes
Famrir's appearance is closer to that of an ancient Asgard than a modern one like Thor and possibly lived in a time before the Asgard used cloning, or before their genomes had degraded to the levels of modern Asgard.