Hi Sam,
You asked about decals for your firefly. Have you decided what unit, and where in Holland you want to model the tank yet?
If you're thinking 'Market Garden' Sep 44 you might want to look at 2nd Bn Irish Guards (Armoured) who lead the advance from 'Joe's Bridge'.
Canadian units were also involved in the liberation of Holland and that would be a different approach.
Re painting, what paints are you using? If they are Tamiya then I use about 50/50 OD and DG with a squirt or two of vallejo yellow, if it's Humbrol then I think the No is matt 155 but check as I'm not 100% sure.
Check out the IWM site for some references or try a google, 'Sherman Tanks in Holland' (images).
You could also look for reference for 30 Corps (XXX) and see what comes up.
Here's the link for the IWM site:
http://www.iwmcollections.org.uk/Good hunting!!!
Cheers
Al