Can I use Firebird database directly from a USB flash disk?
Yes, you can.
Few important tips:
a) Use forced writes. The filesystem cache for USB devices has much higher latency than regular hard disk, so it's quite easy to lose a lot of work in case of power failure.
b) Make sure that your users are aware of consequences of unplugining it while your application is still running.
c) Please note that USB disks are projected with around one milion erase/write cycles. When forced writes are ON, each statement and transaction gets written to the disk, so your users can easily spend around 100 to 1000 rewrites a day - meaning that average USB disk would only last a few years using your application.
d) It is recommended that you maintain a copy of database on the hard disk if possible, or at least force users to do regular backups - the old rule of backup is: backup as much data as you can't afford to lose.
The best practice is to copy the database on the hard disk - work with it - and then write it back to USB disk.