Fiesta Models
Finesse - Mega basic, wide up windows, basic stereo. Avoid.
Firefly - Never heard of it
Studio - Same as style, better exterior (colour coded), basic interior
LX - Basic exterior, well equipped interior (heated screen, AC, etc)
Style - Better exterior, basic interior, has electric mirrors/windows.
Zetec - Has heated screen for the most part, colour coded bumpers, foglights.
Flame - Same as zetec
Ghia - Velour or leather seats, AC, heated/electric everything
Zetec S - Same as zetec except sporty bumpers and 1.6 tdci or 1.6 petrol only. Choice of leather or clothe sport seats.
Silver/Black - Doesnt mean colour, means it has leather seats and AC.
Fiesta Generations (and marks/mks)
In the beginning there was the....
Mark/mk 1 (1976 - 1983)
Mark/mk 2 (1983 - 1988)
Mark/mk 3 (1989 - 1995)
Mark/mk 4 (1997 - 1999)
Mark/mk 5 (1999 - 2002)
Mark/mk 6 (2002 - 2005)
Mark/mk 6(.5/facelift) (2005 - 2008)
Mark/mk 7 (2008 - Present)
It is key to remember that generations are implied by car clubs and enthusiasts. They are not official terminology to Ford, or to parts centres. It is highly likely people outside of the Fiesta community will have no idea what you are referring to when you mention mark 7 Fiesta. It is easier to say the year range.
Interesting to note is that the Wikipedia article which has citations to Ford, states the generations are one prior, i.e. mark 7 is actually mark 8.