Avoiding Infection?

Hi, all,  I have a few questions.  First, what's the best browser to use for security -- not necessarily privacy, but security?  I'm currently using Firebox and Chrome, but am having problems, as you'll see.

 

 

 

     Here's what's happening.  When I visit various websites, for the past few months I've gotten some sort of virus/Trojan/whatever that affects my computer.  This has never happened before; it's a relatively recent occurrence.  Sometimes, this causes Thunderbird to open in safe mode only, or when I click on an icon on my desktop, several icons will be highlighted instead of just one and all open at once; in worst case scenarios, my home page changes to one claiming to be Mozilla's, but with ads and links that I can't eliminate.  I can solve all these problems by using Norton, though I usually have to uninstall and reinstall Norton; the Eraser alone doesn't help. 

 

 

 

     My question: how can I use Norton, another program, or some other strategy to avoid getting these infections in the first place?  (I practice common sense behavior; I'm not clicking on or opening anything or doing anything like that.)  Is there a setting that prevents websites from corrupting my system?  Just so you know, I'm computer literate after a fashion, but probably unable to do anything thing that involves any fancy programming stuff.  Thanks for your advice.  Cheers!