I have a system that is slowing down the longer it stays running and I'd like to know why.

...

Is there any evidence I can gather before I reboot the system which I expect (from past experience) will return the system to the better performance

Hmmm....   If rebooting will solve the problem and you don't have a very long running transaction, then I doubt that garbage accumulation or collection is the source of the problem.  

Nor is there anything else wrong with your physical database - fragmentation or whatever.

I'd look at memory usage - using both Firebird and OS tools.  Look for paging before you reboot.

After you reboot, track memory usage daily or more often.   It doesn't take much of a leak - or unfortunate caching - to built up over month of 27 tps.

Accelerating the move to 2.5.4 would also be a good idea, both because it may correct the problem and because it probably offers better diagnostic tools for this sort of problem.  

Good luck,

Ann