The Rebels were reinstated after their quarter-final conquerors Nantwich were thrown out for fielding an ineligible player.
And FC made the most of their unexpected opportunity with two Chris Baguley goals - one of them a shot from 40 yards - plus another from Rory Patterson taking them sailing through a semi which was spoiled by a strong wind.
FC's opponents in the final will be neighbours Radcliffe, who they also meet in the Manchester Premier Cup semis.
Struggling Boro again shrugged off their poor league form to book a final ticket with a 1-0 win at Ossett Albion courtesy of Craig Dawson's scrambled 26th minute strike.
Meanwhile, to add to FC's joy, their division one north title hopes were also given a double boost as big rivals Curzon Ashton and Bamber Bridge, both lost.
Second-placed Curzon's 2-1 defeat at Clitheroe came as a bitter blow as they went down to a last-minute goal against 10-men.
Jorg Diaz gave Curzon a 20th minute lead, but Clitheroe hit back to deny them a chance to go top.
Trafford maintained their eight-point lead over second-placed Salford in the Vodkat League with goals by Chris Mackay and Kayden Coppin giving them a 2-0 derby victory over Maine Road.
Tweet
Free Red, Manchester (10/03/2008 at 15:16)
Travel: By Car: Leave the M1 at Junction 40. Take the Wakefield road for 200 yards, then turn right at the traffic lights down Queens Drive. At the end turn right onto Station Road, then take the second left down Southdale Road. At the bottom of the hill turn right then left down Dimple Wells.
By Train: The train takes approximately 40 minutes from Manchester Piccadilly to Dewsbury, followed by a 10 minute taxi ride. Buses are available from Dewsbury to Ossett although there is a 10 minute walk either side of the journey. Outbound: 16:27 16:55 17:25 17:55 18:25. Return: 21:51 22:51