However, Mozilla is getting paid for the software. Just because it is open source, does not mean that people don't get paid.
Mozilla takes money for their software in the form of donations. So some users are actually paying directly to Mozilla.
Also, Mozilla makes most of its money by selling private information to Google (searches etc). Users are paying through giving up some of their privacy, by doing searches through Firefox, and google.
Also, the Mozilla foundation is a non profit organisation which means all the US tax payers give it money indirectly through government support. If it was a company, then it would not get certain privileged treatment.
Finally users contribute to OSS through at least testing products - if not in documentation and many other ways.
Anyway, my point is that you can't really use the excuse that it's free to the users, so they can't expect anything - because it certainly isn't entirely free.
People should try and not be rude - on either the developers side or the users side. I've seen a bunch of rants from Mozilla people about this, so I think their community needs work. They should consider watching how their developers communicate with people as well. Calling people 'dicks' and such is pretty harsh. They should also consider making Bugzilla nicer. Things like WONTFIX, and other communications with bug reporters can really make people angry, and can make Mozilla come across as being rude too.