.mbn is Main Firmware, this is like an operating system on your PC (e.g. Windows or Linux)
.fbn is filesystem, this is like your harddrive. It contains various files needed to let the main firmware operate.
You can download them both from
http://share2flash.com/file/6187, or www.iprotebe.cz or www.topsony.com (very good website, has the greatest collection of flash files, but unfortunately it is paid site, but for 2 euros you can get access to download area).
Also, you would better use a program called "XS++". It's very much safer for flashing main and filesystem. I bricked my K750 with SeTool (but recovered it). To get filesystem access and to apply patches, you'd better use JDFlasher.
AND REMEMBER, PLEASE MAKE GDFS BACKUP BEFORE FLASHING YOUR PHONE. IF YOU BRICK YOUR PHONE, AND IT WON'T TURN ON, THE ONLY WAY TO RECOVER IT IS BY RESTORING GDFS BACKUP. And I'm talking from experience. Please remember that you can ONLY flash YOUR backup, not one taken from another person, since it is unique for your phone.Btw sorry for caps, I just don't want these newbies to brick their phones.
Edit: Another tip: use
sswcheck by den_po to check if the downloaded firmware is not corrupted.