Hi Al and welcome to the forum
Did you get a build manual with the Firefly
If you look at the two idler housings at the rear of the Sherman, you will see that the idler is mounted to an eccentric shaft.
This means that, as the shaft rotates in the housing which is bolted to the rear of the tank, the idler moves around in a circle. It is this motion which allows you to tighten up the tracks.
The procedure is this:
1) Remove the large nut from the inside end of the idler shaft.
2) Remove the small bolt holding the plate to the idler housing.
3) Remove the plate from the idler shaft - you should now find that the idler shaft rotates freely in the housing and you can alter the track tightness by varying the position of the idler.
4) Once you have the track tightness to your satisfaction, replace the plate onto the idler shaft so that the slot is over the hole for the small bolt.
5) Replace the small bolt so that the plate is locked into position and the track tightness set.
6) Replace the large nut onto the end of the idler shaft to hold everything in position.
This is best done with the tank suspension compressed, as the Sherman runs best with the tracks quite tight, and this is quite hard to achieve just using the process outlined above.
Be careful when undoing the parts in 1) and 2) as the tracks can hold quite a lot of tension and can bite if this is released in an uncontrolled fashion. BTDT
Adrian