I doubt any bios version supports 5.25 in this model.  flashing backwards to earlier bios version can be risky. you probably need a pentium III based motherboard to support 5.25 floppy, such as Dell xps T700(r).  the spec needs to mention 5.25 diskette such as this one:  Externally accessible bays
OK ... I REALLY understand that I have gone to the prehistoric age of geek here... but I'm trying to help a friend recover data from some very old 5 1/4" floppy drives. I have a Dell GX260 desktop with what (I think) is latest BIOS - A09 I have disconnected the OEM 34-pin floppy cable from the motherboard to the built-in 1.44" 3.5" floppy and have attached a 34-pin ribbon cable that will mate an 5 1/4" drive to the mother board. And also route the 4-pin power connector to the 5 1/4" drive ... On boot up the 5 1/4 spins, but the BIOS will only allow selection of NO floppy or  3.5" - 1.44 disk Q:  Are there any tricks I can use to allow the  5 1/4" floppy to be recognized?   thanks! Jim
Support for 5.25" 5 1/4 floppy drive on Dell GX260 desktop