I installed Acrobat CS3 on my Macbook for testing at work. I did experience the firewall-turning-off issue, but I also had another issue crop up. When opening .PDFs in Safari, rather then the the document opening up in an embedded version of Preview, I was getting a dialog asking me what application to use to open the PDF (which unfortunatley had preview grayed out).
I suspected Acrobat was the cause of the problem, so I opened Acrobat and was greeted with a dialog asking if I wanted to make Acrobat the default application for opening PDFs (I said “no”, but it didn’t seem to take notice). After this, I restarted Safari, and just got a spinning icon in the middle of Safari when I try to open PDFs. I went back to Acrobat and unchecked the “Display PDF in browser using:” option in the Internet category. I quit Acrobat, relaunched Safari, and everything was back to normal.