Brentford bagged a fifth successive victory since Mark Warburton succeeded Uwe Rosler at the helm when overcoming Peterborough.
Forshaw made no mistake from the spot in the 16th minute after Posh keeper had dashed from his line only to take down Saunders.
Donaldson steered a golden chance to double the visitors' lead against an upright before Posh levelled when Jack Payne's dipping 25-yard drive flew past David Button in the 33rd minute but parity lasted just three minutes as Saunders coolly restored the Brentford advantage with a smart, low strike from 20 yards.
Forshaw saw another penalty, awarded after Payne had pulled down Donaldson, saved early in the second half before Donaldson secured Brentford's 12th win in 13 matches in stoppage time by tapping in a Stuart Dallas cross.
Ferguson said: "I don't think I should be under pressure.
"I know there will be fans calling for my head, but I'm one of the best managers this club has ever had and my record here stands up well.
"The challenge is to get out of this league and we have 22 remaining games in which to make that happen.
"The stats don't lie and it is obvious we are in a bad run at the moment, but we are not getting the rub of the green.
"We should have had three penalties in this game yet Brentford were given two, one of which was an absolute disgrace.
"People just need to stick with us and judge us at the end of the season."
- Sports & Recreation
- Clayton Donaldson
- Sam Saunders