Yes. I agree with you. So i did a "make clean" and a "make" (not a make -j3 as i use to do) and recompiled 2.6.25-rc3 and now it works again. Case closed under strange error. Thanks for your support.
My conclusion so far: There is something wrong with your system which is below/ before the control of the ieee1394 or firewire stack.
Yes. 2.6.24 works with the same config, i.e. both drivers are build as modules.
BTW, there was a single change to ohci1394 after v2.6.24 and that does not affect the initialization code of ohci1394. (Patch name is "ieee1394: ohci1394: don't schedule IT tasklets on IR events".) Did you ever use kernels before 2.6.25-rc2 and had both of ohci1394 and firewire-ohci inserted?