"Dynamat" helps in every way. Wherever you apply it, it will have an effect. Doors are generally better for better quality of sound from the speakers. Floor, firewall, trunk, headliner areas are better for lowering road noise. Also, trunk application will also help in a better bass response from the subwoofer.
The cheaper and easier places to do it are the doors, just because they are easier to get to. The whole floor and firewall require a lot of work to get everything out (seats, carpet, dash) which is why most people just do the doors and easier to get to places.
Check Jumbosrule's thread, he did his doors and part of his floor not too long ago.
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Frankiago's thread:
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