Forster nodded home skipper Adam Virgo's cross from five yards for his second of the season in the 68th minute to silence the home crowd.
Brighton - who had shipped 11 goals in their last two games - looked shaky early on and Jason Price capitalised within six minutes. Jimmy Abdou released David Martin down the left and with the Brighton defence absent, ex-Doncaster forward Price tapped home his pinpoint cross from two-yards.
The frantic opening continued and Gary Alexander should have made it two a minute later when clean through, before Liam Dickinson and Dean Cox both fired shots just wide for the Seagulls.
Brighton pressed after the break and two minutes after Cox's chip hit the bar, the visitors grabbed their equaliser.
Abdou went close with a shot a minute after the leveller and Brighton's James Tunnicliffe cleared off the line in time added on, but the visitors held on.