I have Xp Home SP2, Avast! Home & Zone Alarm Free (the latest versions). I have no problems, they work great together!
In fact even with the 15 day free trail of Zone Alarm Pro with the privacy controls on I had no real problem. The pages would load but the Firefox loading icon wouldn't stop spinning. With privacy controls off the icon stopped when the page finished loading.
If you're having problems, maybe you are infected with malware (viruses, trojan horses, worms, spyware, adware, etc.) See my sig below for good freeware to fight the Malware War. I believe I have assembled an outstanding Army. They defend my PC valiantly.