Did I sound like a bot? :(
Yep, it's a modified version of 3.5E.
That's the first edition CS I believe,
the second edition CS is the one we use. They took out a lot of unnecessary skills so there is much less rolling, and checks like searching and investigation are done by the DM, not by the entire group which saves time imo.
I played 5E for 3 years and even if I really enjoyed it, I still get bored of characters quickly because you can't really personalize them the way you want. With a good DM, I honestly prefer Pathfinder over 5E and ours also says he prefers Pathfinder. It shows 'cause he's really experienced with it, so it's been a smooth and really fun experience so far.
If you haven't given the 2nd edition a go you should def try it!