i personally have Ad-Aware and it does wonders for me but i donno about the other 2, i heard nod32 was good but i use avg free anti-virus, i havent had any viruses for months.
Spybot should not have any compatability issues with firefox but if it does I recommend you use Opera. Look in my sig for a link.
Even though talldude123 recommends windows firewall and thinks firewalls are a waste of memory, I still think firewalls are essential. I use Sygate personal firewall.
I do use Opera. Im not saying they have problems together, those are just two programs that never seem to run good on my PC no matter what. Nothing to do with each other
Do you connect to the internet via a router? If yes there is no point using a software firewall.
(Software firewalls actually make it harder for the hacker to get out of your system. The only real advantage is they block ports - which a router does by default.)
I'm not 100% ontop of software, but Talldude will back me up/correct me on this. Sometimes websites cause the firewall to think that it is a threat for some reason or another. May also ports being accessed externally that the router blocks but the request may be transfered to the PC anyway causing the firewall to also try to block it.