You've been lucky ;).
Drivers make assumptions about functionalities present and bugs absent in firmware newer than version X. Only certain combinations of drivers and firmware are tested, because it would be prohibitively resource-consuming to test all the pairs. Also it doesn't make sense to verify new versions of code against an ancient counterpart with known bugs fixed in later releases.
If the server was running for a long time it may be that you've found another working combination or simply lucked out and didn't hit any bugs possible because of the mismatch. Worst case (based on experience with non-Dell hardware): server corrupts data on the disks, crashes and refuses to boot. HW specialist called to repair it will wonder why did it ever run with this FW/driver combination. Best case: server runs without any problems until it is replaced by a newer one 10 years since.
My recommendation would be to upgrade to the next to newest version of firmware and the corresponding recommended driver. This should give you something that is reasonably new and supported, but not cutting edge code which may have yet undiscovered bugs.
Note: Always have tested backups when modifying storage system configuration.