Scott Dobie rounded off a fantastic first-half move to put the home side in front, and Ifill came off the bench in the second half to wrap up the points with a wonder goal.
Millwall started brightest and after Adam Virgo was unable to make the most of a half chance for Brighton, the Lions swiftly moved to the other end and brought the first save from Seagulls goalkeeper Michel Kuipers through Peter Sweeney's volley.
Dennis Wise's side continued to put their visitors under pressure and on 21 minutes they got their reward following a fantastic move.
Danny Dichio played a quick free-kick to Alan Dunne, who in turn picked out Dobie with a perfect defence-splitting pass.
The former West Brom forward took the ball in his stride round the back of the Albion defence and showed great composure to slot the ball past Kuipers.
Brighton almost hit back minutes later. Steve Claridge was a big favourite with Millwall and he came within inches of scoring against his old club.
Charlie Oatway flicked the ball on to Claridge who acrobatically steered the ball goalwards with his back to goal.
Unfortunately for him, Jody Morris was on the goalline to head his effort against the bar and Millwall scrambled the ball away.
The remainder of the half fizzled out, but the introduction of Brighton's Leon Knight on 47 minutes brought the game to life.
Albion had a new lease of life and Virgo and Oatway both forced good saves out of Millwall stopper Andy Marshall.
Despite the pressure it was Millwall who scored the vital next goal to decide the game.
Ifill received a Wise pass on the right and cut inside before bending a superb strike into the top corner for his first goal of the season.
Brighton were given the chance of a last minute lifeline after Knight was brought down in the area by Marcus Phillips.
But Knight's weak spot kick was saved the Millwall keeper.