I�m pretty sure ZA is doing its job as it should be. You have given ZA permission for Firefox to access the net so when you click on the hyperlink all that is happening is that Firefox is connecting to the page you have requested. Another process or application is not getting past ZA. The game popup just contained a simple hyperlink, like all web pages do.
ZA is a very good firewall but if you are worried that your pc has been compromised in some way then do this.
Download and run CCleaner from here
http://www.ccleaner.com/ Its free and removes all temp internet files, cookies, etc. This helps to prevent false positives when running virus and spyware scans.
Go to
http://housecall.trendmicro.com/ This is TrendMicros site, run the free virus and spyware scan.
Download and run Adaware from
http://www.lavasoftusa.com/software/adaware/ again this is free and will check you PC for Spyware etc
There are other free cleaners/checkers if you want some more but the three I�ve mentioned are some of the best.