Without wishing to get into the politics of the browser wars, there's a security incentive in avoiding the critical mass.. Motto: keep away from the most popular software applications that will always be targetted by hackers. For that reason, Microsoft Windows is a no-no for the security conscious, as is anything that ships by default, like Internet Explorer, or Microsoft Outlook, etc..
Does that explain the underlying problem with Firefox? It's not badly designed as such. It's just that as it gained popularity, it fell into the cross-hairs of the same 'black hats' who nobbled Explorer. Maybe Google Chrome, as it gains market share will suffer the same fate? There have already been a few exploits for Chrome. Google PR tried to turn them into positive events, by rewarding the hackers. However, the truth is that the Chrome codebase isn't so solid after all.
Gecko, the underlying rendering engine of Firefox is blisteringly fast. It's all the chaff in Firefox that slows it down. Several browser projects are based upon Gecko but don't have that Mozilla chaff and they 'go like the clappers'..
The Galeon browser was my favourite stripped-down Gecko-based browser project. It was brilliant. Sadly, like so many open source projects it fell by the wayside once the developers graduated from university and moved into commercial work. Even Galeon's load time was amazing. The browser binary was only a couple of megabytes, so it took milliseconds to load from 'cold' on a desktop PC.
All browsers are probably fairly secure, the problem is more often with the plugins - especially the closed source plugins for things like Flash. That is a security nightmare!
cheers, a