Definitely check US Airways as they fly direct from Manchester to Philly (I'm sure you know) - and last I knew, they have one of the flexible dates thing search features which shows you which days/dates are cheaper to fly, etc.
However, the time period you are looking to go appears to be in the midst of school holidays here, which usually adds to the price!
(especially for flights to Florida, which I'm not sure are ever cheap anyway!)
I hear good things about:
http://www.skyscanner.net/We usually find a lot of the airfares (for our route, for the exact dates we want, etc) much of a muchness, I surf around different airline websites & then we pay what seems to be the going rate for the time. I have yet to find flights at real 'steal' but we usually aren't very flexible about when we can travel either. YMMV.
I'm sorry about your dad, Andee.
If you're off this Monday, let me know - making plans to get together with BostonDiner then.