Reasonably difficult, I think. I’m basing my answer off of the vibes I get from open source firmware projects for routers, which are far more common. I haven’t heard of similar for printers, which suggests that there is less of a foundation to work on. I think Brother, in particular, was a brand that has typically been decent up until now.

I also get the sense that programming firmware is different enough to programming software that a software developer trying to contribute would find it really difficult(?. Someone correct me if I’m wrong — I’m not a software developer, but a scientist who writes code, so I’m speaking outside of my main expertise). But this loops back in with the lack of existing projects making it harder to get to grips with how to do stuff — part of why I like open source programs is because I can look through a project and try to understand what the code is doing.