A blistering seven-goal second quarter laid the platform for the Dockers as their key position players, led by Josh Treacy and Alex Pearce, starred in the 17.10 (112) to 14.13 (97) victory at Marvel Stadium on Sunday.
Fremantle (16-7) could have finished as low as ninth with a loss, but will now host an elimination final in week one.
"We seem to play our best footy against the better teams, as you saw for periods of the game tonight," Longmuir said.
"We've had seven wins against the top nine sides now and our players will get a lot of belief and confidence out of that.
"The contested game seems to suit us, so I'm looking forward to it."
The Bulldogs face a nervous wait to learn their finals fate, which will be determined when Gold Coast host Essendon in the last game of the home-and-away season.
If the Suns avoid defeat on Wednesday night, they will replace the Bulldogs in the top eight.
Bulldogs coach Luke Beveridge felt the Dockers' size and strength at both ends was the difference on Sunday, and conceded his side had almost certainly blown their finals chance.
"There's a lot of disappointment in all of that," Beveridge said.
"Everyone's as flat as a shit-carter's (hat), there's no doubt about that."
Fremantle's talls were outstanding in their crucial win.
Pearce was superb in blunting Bulldogs spearhead Sam Darcy and Brennan Cox kept Aaron Naughton (two goals) mostly quiet.
At the other end, key forwards Patrick Voss, Jye Amiss and Treacy had three goals each.
Caleb Serong (24 disposals), Jordan Clark (23), Shai Bolton (19 touches, two goals) and Neil Erasmus (20 touches, six clearances, one goal) were also important.
The only sour note for the Dockers was Corey Wagner tearing a pectoral muscle, an injury which appears set to sideline him in September.
Longmuir expects Sam Switkowski to play in the first final despite a late knock to the small forward's knee.
The Bulldogs failed to make the most of their chances when they controlled play in the opening term - leading 4.5 to 2.2 at quarter-time - and it cost them dearly.
Fremantle seized control with seven unanswered goals in the second-quarter avalanche and fought off repeated challenges in the second half.
Longmuir was proud of his players' response after they had come under an intense spotlight following the previous week's home thumping from Brisbane.
"From the start of the week I felt like we re-set," Longmuir said.
"We didn't catastrophise it, we looked into it and we got back on the horse pretty quick.
"We straightened up with some pretty simple focuses and I felt like there was that sort of performance coming all week."
Bulldogs skipper Marcus Bontempelli fought hard in a losing battle with 33 disposals, 11 clearances and two goals, and threatened to drag his side back into the contest late.
Tom Liberatore (27 disposals, eight clearances) also toiled and Joel Freijah finished with three majors.
