Although no-one in the Ton camp will concede defeat until it is mathematically impossible, the Jags' 1-0 win means they have all-but secured the title and promotion barring a catastrophic collapse.
The triumph, courtesy of a James Craigen goal just before half-time, pushed Thistle five points clear. And with a game in hand and just four games left to play afterwards, it looks all over bar the shouting.
The match was, quite rightly, billed as the biggest match of this season - and most seasons in the last quarter of a century, for that matter - but the action failed to live up to the hype.
Fans of both teams had already started queuing outside the ground over an hour before kick-off, and a fantastic crowd of 8,857 would eventually pack the old Maryhill venue to the rafters.
That was, however, after kick-off was delayed by 15 minutes to allow the throngs gathered outside time to filter into the stadium.
Meanwhile, inside Firhill the players were going through their pre-match rituals, three of whom were returning to action after absence.
Martin Hardie was back after a one-match suspension and took up a place at right midfield, while Kevin Rutkiewicz and Thomas O'Ware both returned after missing the Airdrie win through injury.
Allan Moore decided to drop Scott Taggart and play O'Ware at left-back in order to move Craig Reid to right-back and accommodate Kevin Rutkiewicz and Mark McLaughlin in the centre.
The reasoning would have been that Taggart's swashbuckling style could leave Ton exposed at the other end as Thistle raided while he was caught up field.
O'Ware, meanwhile, has been a strong performer whenever he has been called upon this term. The fact he hadn't let him down would have reassured Moore he could rely on him in such a big game.
Nevertheless, it was a big call, and something that, with the benefit of hindsight, the Ton boss might have done differently.
The atmosphere was electric when the match eventually kicked-off just after 8pm, and both sides seemed to be stifled by magnitude of the occasion.
There was a nervous energy that was tangible, so thick in the air that you could almost cut it with a knife. Neither side wanted to risk making a mistake and they both played it safe as a result.
Ton tried to start by firing balls into the corners and getting their hosts turning. They were also trying to be the aggressors, getting in Thistle's faces in order to stop them settling.
Midfield hardman Hardie put in an early 'reducer' on flying Thistle full-back Aaron Sinclair, which earned him a talking to from referee Brian Colvin.
The visitors were the first to threaten in front of goal when, on 16 minutes, Peter MacDonald challenged for a Derek Gaston punt, helping the ball to run through to David O'Brien.
Wideman O'Brien burrowed his way in off the left wing and into the box before placing a tame right-footed effort at goal which Scott Fox gathered with ease in at his midriff.
Morton were doing reasonably well to stop Alan Archibald's men from finding their rhythm. Rutkiewicz was right in behind Chris Erskine to get a toe in and upset his game. On the one occasion Thistle really threatened in the first half hour, Stephen O'Donnell's cross from the right was mopped up by McLaughlin.
While not fairing a whole lot better in front of goal, or playing much if any coherent or connected football, Ton still looked slightly more likely to score at this stage.
After a tidy turn round his marker, Fouad Bachirou made a beeline for goal and drilled a speculative drive just wide of the left-hand upright. It didn't hit the target but it was encouraging.
But the wheels started to come off Ton's high intensity start around the half-hour mark as Thistle began to get a foothold. Sinclair was getting a lot of space to advance unopposed down the left and fired over a cross which floated over Gaston but fortunately landed on the roof of the net on 32 minutes.
He then repeated the trick one minute later but produced a better final ball, picking out Kris Doolan, whose glancing near-post header was parried by Gaston.
Sinclair's opposite number, O'Ware, wasn't fairing quite as well in possession. Although he was excelling defensively, winning headers and popping up to clear, he was a bag of nerves on the ball.
More than once he sent it out of play or aimlessly down the line leaving Peter MacDonald with no chance - and those precedents might have been in his mind on 41 minutes. Instead of going up the line, he shifted the ball back onto his right and got caught in two minds, hitting a mixture between a crossfield pass and clearance. It was neither and gifted possession straight to Thistle. And the slip was ruthlessly punished when Erskine picked out Craigen with a low cross from the left and he swept past Gaston at the far post.
O'Ware didn't appear for the second half and was replaced by attack-minded full-back Taggart, who went to the right while Reid switched over to the left.
And Taggart almost made an instant impact, seizing on a poor Craigen pass, which was remarkably similar to O'Ware's, and driving into the box.
But the former Hibs defender lacked composure at the critical moment and fired high over the top and into the raucous Jags fans behind the goal.
Those among the sizeable Ton contingent in the main stand, on the other hand, were reserved with their team giving them little to cheer about.
Tidser wasn't his usual self in midfield, and his sub-par performance was typified by his surprisingly poor set-pieces.
For the most part, Ton were simply lumping balls forward to a forlorn looking MacDonald, who appeared incredibly frustrated figure with the service he was being offered.
On a rare occasion he got the ball into feet on 66 minutes, MacDonald fed Bachirou with an intelligent angled pass in behind the home defence on the inside-right channel. But the promisingly positioned Frenchman couldn't find a team-mate with his cut-back, and Thistle swept clear.
Ton were pinning Thistle back but they were also struggling to create anything meaningful in the final third.
On 70 minutes, Reid sent a weak curling effort well wide, and O'Brien won a corner which came to nothing after MacDonald slipped him in.
For all Morton's huff and puff, the hosts actually had the best effort of the second period when Doolan tested Gaston with a firm 25-yard strike.
The Greenock men were drinking in the last chance saloon, and Moore withdrew McLaughlin, who appeared to tweak his hamstring, and replaced him with Kyle Wilkie in order to move to a 3-5-2.
But it didn't make any real discernible difference as his side failed to manufacture an opening of note in the final quarter of an hour and the Jags held on for a pivotal three points.
In the end, it wasn't the spectacle that was expected. Neither side played particularly well; they seemed to cancel each other out.
The biggest disappointment for Ton is not simply the fact they lost a win or bust match but that they didn't perform to their potential on the biggest night of their best season in 25 years.