I haven’t looked for the article but I believe that Symantec reported that last year Mozilla Firefox had over 120 security holes found to IE’s 50 some. However, not sure of the relative severity of them. It’s increasing popularity will by default make it more targeted. A quick search brings up millions hits many like this one.
http://arstechnica.com/journals/linux.ars/2008/12/17/firefox-security-updates-ready-for-downloadAlso many people think that they are safe just by running Linux but don’t have a clue how to set it up and secure it properly leaving them even more potentially vulnerable than people running windows. Home users just aren’t as targeted yet because the install base is so small compared to other OSs so most of the people cracking systems running Linux are targeting bigger fish like university departments where they can gain access to higher bandwidths and faster computers.
I’m not trying to be a Microsoft fanboy just trying to point out that just because it’s not Microsoft doesn’t mean it’s always safer.
