Their 19-7 victory was No. 8 in a row -- their longest streak since the 1985 Super Bowl team started 12-0.
"All the years I've played against them, I think defensively this is as well as I've seen them play," said a banged-up Favre, who had swelling in his wrist and a cut on his hand after hitting it on the helmet of a Bears' defensive linemen while delivering a third-quarter pass.
"They're just good."
Charles Tillman and Nathan Vasher had game-changing interceptions against Favre, who completed 31 of 58 passes for 277 yards against a team he had dominated over the years.
Tillman returned the first interception just before halftime 95 yards to set up one of Robbie Gould's four field goals, and Vasher took a late interception 45 yards to seal the victory.
"With Brett Favre, you have to read your keys," Tillman said. "He could be getting sacked and still throw it 30 yards. It's kind of hit or miss with Brett Favre."
The Packers (2-10) were driving for the lead near the end of the first half when Tillman intercepted a poorly thrown pass by Favre in the end zone and raced 95 yards before Tony Fisher saved a touchdown and knocked him out at the 7 with six seconds left in the half. Gould's third field goal, a 25-yarder, gave the Bears a 9-7 halftime lead.
"It was hard on the hamstrings; that's all I can say," Tillman said.
"It was a great play," Vasher said.
But with Brown in his face, Favre lofted up a weak pass for Robert Ferguson and Tillman picked it off easily.
"I was in good position," Tillman said. "He just kind of underthrew his target."
Favre said he was trying to throw the ball away but didn't get enough on it as he was being pressured.
"They had the right blitz on for the right play," Favre said, adding that the initial play was for a shovel pass that didn't materialize. "I tried to turn and throw it out of the back of the end zone. As I was throwing, I got hit and couldn't put enough on it. It was a huge play obviously."
Trailing 12-7 late in the game after Gould missed a field goal, the Packers tried to mount a final drive when Favre hit back-to-back passes.
But Vasher -- who earlier this season returned a missed field goal 108 yards, the longest play in NFL history -- was there again, stepping in front of a sideline pass for Donald Driver and racing in for the TD with 3:06 left.
"I had a pretty good read," Vasher said. "We knew that going in he had thrown 19 interceptions prior to today. We knew that he's one of those guys that'll come out and throw something up. That's just his attitude. But he can make plays at the same time. It's just more or less picking opportunities, when we want to try."
Green Bay, already ensured of its first losing season since 1991, had the ball for nearly 10 minutes longer than the Bears, but couldn't score in the second half.
Chicago's Kyle Orton was 6 of 17 for 68 yards, while Thomas Jones ran for 93 yards and passed 1,000 for the first time in his career. The Bears didn't convert a third down in the game (0 for 10) and Muhsin Muhammad had no catches for the first time since 2002, a span of 51 overall games and 47 in the regular season.