Harry Redknapp's side were looking for a fifth straight win but wasted a string of chances to secure it with Charlie Austin the main culprit.
They had won their previous four matches 1-0 but could not even muster the solitary goal which would surely have seen off a Brighton side who are hardly prolific themselves.
With Bobby Zamora and Andy Johnson sidelined it was left to Austin to shoulder the goalscoring burden.
And the fact Rangers ended the night with Armand Traore joining the attack showed why Redknapp wants to bring in another striker on loan.
Stephen Warnock had been sent off for conceding the free-kick and from it, Royston Drenthe found Le Fondre who undid his old boss.
McDermott - sacked by Reading in March after more than 12 years at the club, mostly as chief scout but also as a title-winning manager - received a warm welcome back from the home fans, as did former striker Noel Hunt.
The pair had nothing to celebrate, though, as Nigel Adkins' Royals got a win they just about deserved.