if it's a 256M, it is enought.
also, if it comes from the machine itself, ther is no incompabilities issues.
as those two thin,gs cannot be the cause of the trouble, you may check :
- how did you create the CF ? copy an image with a 'dd' or a 'cat' command ?
- can you check if image is well copied on the card ?
- are you sure the partition is bootable (normaly yes if copied from an image file)
- did you notice some messages on serial port ? are there some error messages
one current issue is a disk name error. if i remember, images are written for a cf in /dev/da0, in your system it may have a different name, so you have to fix the /etc/fstab accordingly to the name given in your machine.
by examining the dmsg or the startup messages, you can check the real name to be used.
what i recommend to you to go further:
the firebox is a blind box, so you do not know what happend, ther are 2 possibilities :
1- check the CF with software on another PC (needs the CF/IDE adaptor, or a CF/USB if the PC can boot on usb disk) - like that you can check if CF is ok, if it is ok, you are sure the problem is above, so in all cases you are obliged to go to step 2.
2- have a look in what is happening inside the firebox:
- connect a null modem cable between the serial port of the firebox and your pc, and look at the ouput with a terminal software (linux minicom is nice for that) if there is some.. (there are messages only if the machine can boot)
- add a video card to see post messages, including eventual problems before/during booting.
I do not know this machine, internal pictures are welcome!
I know that firebox x500 and above series have a PCI slot so you can put a pci graphic card, i do not know if firebox x550e have the same?