I'm going to kick in a few others that should be obvious, but I've been on the 'net since 1995, and I still managed to step on a few toes when I got here. :) NOTE: Not a dev. I have no affiliation with this project, other than I like to use it.
Always be polite. Always! No exceptions. No one likes a jerk. :)
Remember that everyone here is donating their time. No one here is obligated to help you. When you remember that if you spent your free time constantly fixing other people's problems and those people DEMANDED it of you….well, you wouldn't be very happy, would you?
Give back when you can. If you use pfSense for a while, you will naturally start to learn some of the common issues people come across. As you see problems come up that you know the answer to, say so!
Try not to thread hijack. Guilty as charged here. It's really easy to stray off-topic on a thread and take over the topic. Aside from annoying people, it makes it less likely for someone to see your question and answer you. I've done this a few times myself by accident. Make efforts to avoid it. :)
Finally, and again, not part of this project, if you can't easily give back with knowledge, give back with donations. I'm sure these guys appreciate anything that the users are willing to kick in. Heck, $5 goes a long way toward their beverage of choice during a coding session, and believe you me either alcohol or caffeine is WELL appreciated during a coding session!