Again, I realize there are many who find that level of angst exciting and riveting . . . but I honestly want both scenarios (Traveller and Serenity) to move past the mundane and into the realm of Adventures.
		
		
	 
Fox,
You're failing to comprehend 
Traveller on a fundamental level, which is understandable considering that you're confusing 
Serenity with 
Traveller and are only familiar with Mongoose's rather substandard version of the game.
Making monthly payments never were the end all and be all of 
Traveller.  Instead, monthly payments were a 
built-in hook specifically designed to lift the game from the mundane and into the realm of adventure.
The size of the ships, their monthly operating budgets, and their expected monthly incomes were all carefully balanced to push the players towards not only speculative trading but other risky - and thus 
adventurous - activities.  If Eneri, Joe, and Zack could make their "nut" schlepping widgets and passengers between Efate and Regina, they wouldn't be working the streets of startown looking for their next big score.
Simply put, that monthly mortgage payment is the doorway and not a barrier to adventure.
That being said, 
Traveller creators took care to provide other avenues for play apart from the 
We're Going To Get Repo-ed If We Don't Make Our Nut, So Let's Take That Charter Offered By the Crooked Archeologist trope.  And, because, the basic game was built around mortgage payments, nearly all of the these other avenues involve fiddling with mortgage payments in some manner.  Here's a brief list:
Old Ship:  The mortgage has been paid off sometime in the past and the ship is now clear.  Wise GMs balance the lack of a mortgage with increased maintenance requirements, however, as it's always smart of have some sort of a "hold" on the PCs.
Other Owners:  The mortgage is being paid by someone else. someone who the players are working for.  One example of this is the classic 
Subsidized Merchant campaign for which a specific ship design exists.  Others would include working for trading companies, megacorp subsidiaries, or even uniformed services like the Scout or Navy.  The fact that the players will be beholden to a third party gives the GM a nice bit of control over the course of any adventure.
On The Lam:  In this instance, the players have simply stopped making payments and have (hopefully) bugged out for parts unknown.  GM control in this circumstance appears in the form of repo men and bounty hunters, both of which make for nice long term "villains" in your campaign.
As you can see, with just a little thought about various ways to lessen or remove mortgage payments, a GM can trade one hook for another and thus skew his campaign towards a situation he and his players will be more likely to enjoy.
One final word about 
Mongoose Traveller, or 
MgT, and 
Serenity.
First, 
Traveller has been around for over thirty years and there are many more sourcebooks available for the game than just those published by Mongoose.  In fact, Mongoose has yet to publish the definitive version of any of 
Traveller's many aspects; aliens, trading, warfare, exploration, empire building, and so on, and instead has produced rather shoddy examples of the same.  Mongoose's sole claim is that it's materials are new and somewhat relatively easier to find.
If I were starting a campaign however, I would look at both the FFE's Reprints and the SJGame 
GURPS:Traveller line.  You can purchase a CD-ROM from FFE which contains nearly the entirety of 
Classic Traveller, or 
CT, for about the price of two Mongoose splat books.  With that sole CD-ROM, you'll have all the materials that originally set-out 
Traveller's style and background.  Similarly, you can purchase .pdf versions of 
GURPS:Traveller, or 
GT, online and get what is believed to be the definitive economic and exploration systems for 
Traveller in 
GT:Far Trader and 
GT:First In.  Nothing Mongoose has produced and, sadly going by what they've published so far, nothing Mongoose will produce can match 
GT:FT or 
GT:FI.
The 
MgT RPG system is serviceable enough so, if you take the time to apply good sourcebooks to that system, you should have the beginnings of a good game.
Firefly and it's movie 
Serenity are wonderfully evocative and it's a shame we'll never get to see just what their creator had planned for them.  While Mr. Whedon has all but stated he was inspired by 
Traveller when developing 
Firefly/Serenity, we need to remember that he was 
inspired by Traveller and was not 
using Traveller.
So far, I've found that set theory is the best way to get this idea across.  While the 
Traveller set and the 
Firefly/Serenity set do intersect, neither is a subset of the other.  We've had well-meaning, but poorly informed, people arriving at 
COTI assuming that 
Firefly/Serenity is Traveller and, more strangely, that 
Traveller is Firefly/Serenity despite predating it by over a quarter century.  One recent poster actually stated that 
Traveller had a "western feel" to it thanks to their confusion over this issue.
There are portions of each setting that are explicitly at odds with the other; the FTL comms in 
Firefly/Serenity are just one example.  You can use 
portions of 
Traveller to play 
Firefly/Serenity, you cannot use 
Traveller straight out of the box to do the same.  
Good luck with your campaigns and please let us know how they turn out.  
 
Regards,
Bill