Unless this is a very low level package, I find it hard to believe it just doesn't work with Firefox in 2023, at least if we are referring to the basic browser APIs for rendering, interpreting, sandboxing, etc.

In other words, if they aren't doing something systems level that exposes significant differences in the underlying browser APIs, then what's the cause of the issue?

Most application level differences (with the exception of the adware) are very very minor. That didn't used to be the case, but it certainly is today. I.e. you would have to put some effort in/intentionality is usually required to break something in Chrome but not Firefox and vice versa as a standard user program. Its not recommended, but I rarely see front end folks doing the same kind of browser tests that used to be industry standard these days, because it simply isn't an issue and if it is, it's because you are knowingly using a non-standard feature (which is usually adware related). The fact that they figured out the cause so quickly implies it's something like that. Systems level issues would require significant time to investigate and probably the involvement of experts in modern browser implementations.

Just my two cents as an ex front end guy.