3.11.00 firmware

I feel certain that Apple would have built in a safeguard if there were any constraints on updating certain 5s models. We upgraded my wife's 5s with no knowledge of her modem firmware, nor have I ever heard anyone suggest that you should care about this (except you, of course 🙂).

Yes, it is safe

Those numbers do not mean anything if you do not know the code system. They are not simply incremental. My iPhone 6 is 2.something. The numbers are a code that links the actual Qualcomm hardware chipset with a particular iPhone generation or production run and the version of iOS it shipped with. And whatever it is it simply doesn't matter when it comes to upgrading (unless you have jail broken your phone and changed the modem firmware, in which case you are on your own).