After reaching the halfway mark, the athlete pulled up complaining of breathing difficulties in course temperatures of 40C plus.
Only 25 riders out of the 133 starters finished the endurance race.
Last week his state-of-the-art £6,000 time trial bicycle fell from the roof rack of a car and was crushed underneath the wheels of a lorry on the motorway.
The 33-year-old, Jersey’s flag-bearer, has also had to cope with the tragic death, just last month, of his good friend and cycling team manager Tony Richomme.
Firby was in the main peloton, the chasing pack, for his seven laps of the circuit, which was about two minutes behind a small group of early leaders.
The peloton covered places six to 65.
Afterwards he said: ‘I was in a good position and enjoying the race, but then the group upped the tempo and I started to have a problem with my breathing, especially on the two climbs of the circuit.
I kept pushing myself as best as I could, but the elastic eventually snapped.’ Looking at his future involvement in the sport, he said: ‘To be honest I am looking forward to seeing my family and being able to get home and gather my thoughts and have a think about what to do with myself.
Hopefully I will be taking part in the Island Games, but this is certainly my last Commonwealth Games.’ The race was eventually won by Australian Matthew Hayman in time of 4 hr 05.09 min.
Team manager Toby Aubert added: ‘I think his performance, under the circumstances, has been huge.
He’s had to deal with the tragic death of Tony, the loss of his bike and a cold.
It would be fair to say that he has been mentally distracted by all that has gone on.
‘But despite all that, he has tried his best and you can’t ask for any more than that.’
