Firrito gifted Geelong's Jimmy Bartel the game-winning goal in the Roos' four-point loss to the Cats on Sunday, when he kicked the ball away from a pack after teammate Nathan Grima had been penalised for pushing Bartel in the back.
The resultant 50m penalty took Bartel to the goal-line, with his subsequent major putting the Cats in front with just 25 seconds remaining in the game.
In the opening minutes of North's 16-point round-one loss to Collingwood, Firrito conceded a 50m penalty that allowed Steele Sidebottom to kick the game's opening goal from point-blank range.
The veteran Roo again kicked the ball clear having failed to realise Daniel Wells had been penalised for chopping Sidebottom's arms.
Firrito told NMFC.com.au on Monday he'd had a sleepless night after the Geelong loss as he tried to come to grips with his costly mistake.
"It's been 20 hours now but it's still a pretty sickening feeling in your stomach and the one that got away, so we are bitterly disappointed.
"It's one of those things you think about. I didn't get too much sleep last night, going over it in [my] head.
"It's two (50m penalties conceded) in two weeks for me, pretty much on the same bit of Etihad (Stadium), so it's something I'm going to have to sum up.
"I'd be a fool to say it's just luck and it's just happened two weeks in a row. It's something I'm going to have to address and be really careful of."
Although admitting he had no excuses for the Bartel 50m penalty, Firrito said he hadn't heard the umpire's whistle when the free kick was paid against Grima.
"It was pretty loud there … I need to sum up the situation a little bit better.
"There was a lot of tackles in the last quarter and the umpire was letting a lot of it go. I didn't hear anything and I just wanted to clear the area, but if I had my time again I'd certainly be holding onto that ball."
Firrito said North had taken a lot of positives out of its first half against the Cats, when it led by 41 points late in the quarter before taking a 35-point lead into half-time.
But he said North's post-match review had identified a lot of things that had gone wrong in the second half.
"We really respect the Geelong Football Club and we think they're going to be a very good side this year, so to put ourselves in that position was great, but what went wrong we certainly looked at (on Monday) as well," Firrito said.
Firrito said the Roos' clash with the Sydney Swans at Hobart's Blundstone Arena this Saturday would be "a massive game" for the club given its 0-2 start to the season.
"The strength of character we've got amongst the boys will hold us in good stead, and we'll bounce back," he said.
Nick Bowen covers North Melbourne news for AFL.com.au. Follow him on Twitter: @AFL_Nick
