I'm planning to debrand the Sony Ericsson Z520a phone, but the firmware files I found online for it do not match the current firmware perfectly. They are both for the same phone and they are both CID 36, but the downloaded firmware is R3F015_RED36 while my phone's firmware is R3F017_RED36. Does this matter when flashing the phone? Any help and/or suggestions would be greatly appreciated.

R3F017_RED36 is probably the branded version of R3F015_RED36.

It should go in fine: just be aware that when you flash it you will lose everything thats on your phone memory (including your contacts, MMS settings etc) so back up what you can. If its unlocked already theres also a chance you could lose that as well (but only if its patched unlocked: if its permanently unlocked then you wont