Thanks to j.p.s, I found out that 5.10.3 also belongs to this first firmware group, meaning that instead of 5.11.1, now 5.10.3 is the lowest supported version.
My script that downloads and analyses the different firmware versions didn't download that version for some reason, but after I manually did, it categorized it in the first group. It's already been tested and confirmed to be working.
Now if I can fix up the jailbreak code, (which I was told is semi-outdated even though it worked for every version we've tested it on) I'll make the thread.