As others have said, a large number of Tau Fire Warriors should be an intimidating obstacle for space marines. Anyone stupid enough to try and fight them in a straight shooting match will get mauled unless they devastate the Tau before they open fire. You deal with Fire Warriors by not letting them get that chance to shoot at you. Take cover, sneak up on them and destroy them with a hail of fire or charge into melee. The Tau are powerful at range but in melee they're even more feeble than a PDF trooper.