The main difference, Tech wise at least, between Firefly and Traveller is the drive system. Firefly using a "pulse drive" and ships are rated for speed and range (in hours). So, if you want to run a Firefly style game using Traveller rules or if you want to use Traveller ships in the Serenity RPG, it is not that hard to convert them over.
By doing a little comparison of the rules I think Serenity ship tonnages are (roughly) 4 times larger than Traveller tonnages (this is based on digging into the description of crew quarters, witch reads almost the same as LLB2 in a couple of key sentences except that minimum space for a crew member is 8 tons rather than 2[double occupancy cabin]).
So, here is my formula for those who want to turn a Traveller jump drive into a Firefly 'Verse pulse drive.
Take the jump number and multiply it by 2. This will give you the ship's Firefly speed class.
Next, to figure out the ships operational endurance in hours, you have to work though the following formula:
Traveller fuel tonnage x .8 = Firefly fuel tonnage
(I know I said that Firefly tonnage was 4 time Traveller tonnage, but Firefly fuel tanks are also 5 times larger than the actual fuel tonnage they carry.)
Next:
Firefly fuel tonnage / Speed Class
---------------------------------- = Endurance mod
Firefly hull tonnage / 100
Next:
Endurance Mod x 600 hours (which seems to be the base standard in the game).
This gives you the number of hours of flight endurance for the ship.
The Serenity RPG measures distances in time and has a table that gives you time ranges for travelling between moons and planets, planets in the same system, and planets in adjacent systems, all rated in hours of flight time for a Speed Class 1 ship.
Running the numbers on a Book 2 Type S gives you 1,200 hours of flight time, and on a Type A it's 900 hours.
Just though I would share...