What it mostly comes down to is that communication several times seemingly seemed a one way street. Where I provided information (often explicitly asked for) only to be effectively ghosted. Not in the sense that I was dealing with busy people where it just took time for them to get back to it again. But really getting no response at all. Often when I then did follow up on it (several months later) I would see the bugzilla ticket change a tag or some other meta attribute but nothing more.
To be clear, this isn't even unique to Mozilla/Firefox. I had similar experiences on other open source projects, although it differs really per project. It is more that with something as big as a browser, where setting up the development environment can already take up the better part of a day, it becomes an extra barrier for even trying.
