Furthur is canceling a tour date after guitarist Bob Weir fell onstage at a concert last week. Weir, 65, said on his website Tuesday that the Grateful Dead offshoot is ‘‘unable to perform for the next several weeks . . . due to unforeseen circumstances.’’ They were scheduled to play May 9 in Napa, Calif. Weir was quickly helped off the floor after collapsing onstage at a concert last week in Port Chester, N.Y. Weir formed Furthur with fellow Grateful Dead member Phil Lesh. Furthur’s summer tour kicks off July 11 in Brooklyn, N.Y.