Hiya - my work involves writing JavaScript active content, mainly for an Intranet of around 60,000 desktops (UK Govt, taxes). I am mostly involved in the R&D stages - I develop something, someone else gets the credit - that sort of thing. Not too suprisingly, the lack of imagination in the organisation means they run IE.
Being an active content writer, I've developed stuff for my own websites:
http://www.coxellis.freeserve.co.uk/
http://www.coxellis.pwp.blueyonder.co.uk/
In the first of the above two links, the whole start page was an experiment in running pages entirely as Javascript library objects. In my astronomy pics site, I can just update an array with a picture's details and it's added.
I doubt if either page runs properly in firefox. I tried firefox out a while back and had problems with missing plugins and limited support for active content, so I switched back to IE. TBH, I could (and should) re-write the pages to support FireFox, but I'm just too busy...
I don't have an issue with security, my main computer is the laptop I'm typing this on and it is firewalled behind a router (NAT firewalled) and has it's own firewall anyway and I run a WEP based wireless LAN. I also use up to date virus software
So there you go.
I'll try it again sometme in the near future, but tbh, I manage fine with IE
Generally speaking though, I'd agree firefox is the better product.
Cheers
Andy