I myself programmed some malware for Linux, in FreeBASIC. It is harmless
but annoying. It has to be started manually by double-clicking the icon and you
can get rid of the effect by restarting your PC. Nothing serious. If you are
interested I will send you some versions. As for Firefox malware, I once
installed Windows live search (it can be installed in Firefox under Linux and
cannot be uninstalled) and got rid of it simply by deleting the .mozilla folder
under Slax.