As mhallbiai said its probably people who use your ISP have caused the whole IP block to get flagged. Its unlikely to be firefox doing anything - but you could always close firefox and browse there with konqueror instead and see what happens. Its also extremely unlikely that you have a Linux virus as most are either proof-of-concept only, exploit security holes long since closed (esp. in a brand new version of Suse) or simply never spread around in any significant numbers. There is also no known spyware for Linux at this time.
Just put up with having to type the image verification thing to access google. Then complain bitterly to your ISP. If their network is riddled with Windows users infested with virus/spyware then its their responsibility to cut those users off until they clean their machines. Or just change ISPs