We are developing irule on QA F5 which is running on version 22.214.171.124 and need to replicate/apply the same iRule in production F5 which is running on version 126.96.36.199. Does differ of firmware version will impact the iRule functionality?
The advice that Mohamed_Ahmed_Kansoh gives here is solid, you always want to check the bugtracker and release notes to be sure.
That said, if you are developing iRules on an earlier version like 15.x, for the most part they should work as is on 16.x/17.x. It's not so much the case in the last several years, but early on in the TMOS era, there were significant changes that could potentially break your iRules or enhancements where you'd want to rewrite them to take advantage of the updates. But between these versions, neither of those scenarios is likely to impact you.