To my knowledge, any fire-related protection will be fairly expensive. Beyond attaching your UPS's emergency power-off (EPO) functionality to your building's fire suppression system such that the EPO is tripped when the fire suppression system kicks in, I don't think there's much more you can do in a small office for reasonable money.
Protecting against theft to any great degree is also going to be expensive. Hopefully your server is covered by the physical security mechanisms already in your building-- alarm, cameras, motion detectors, locked doors, etc. To secure the box itself I've used padlock hasps or Kensington lock attachment points that some server computers come equpped with to secure them from being opened and moved using steel cables (basically, glorified bicycle locks). If your server is in a rack cabinet you can use the built-in locks, but beware that many manufacturers (I'm looking at you, Dell, especially) use the same key set for nearly all of their cabinets. (I love unlocking the racks at sites I've never been in before with the one of the variety of rack keys I carry...)
Hedging your bets against theft using off-site backup and full disk encryption, along with insurance for the physical hardware, are your best bets. You're certainly already on the right track w/ your use of off-site backup.