First off, uniforms not liveries. I have never tried doing this with the firefighters, but i got tired of seeing cops with the same name tag, breast bars and badge numbers, so what i did was i made more uniforms and put them into openiv, so what you do is find the type of uniform you want to edit in the wtd file: 000,001,002... and then look for what letter of the alphabet is being used after the number: a,b,c,d... So for me i wanted to make more long sleeved cops. So i went into cop.wtd and exported uppr_diff_001_a_uni.dds to the desktop, edited the uniforms a bunch a times and saved them a bunch of times, then imported them as uppr_diff_001_b_uni,
uppr_diff_001_c_diff, and so on and so forth. Hope this helps.