You might like to check out the tech list - link is at the bottom of the left-hand column.

You will need to say which version of FireFox you are running and on which operating system.

I am running version 1.0 Preview Release on Suse Linux pro 9.2 and images are displaying correctly.

However, I once used another version of FireFox on another OS and some Indy sites just got displayed as a collection of chinese characters.

For more about firefox see
 http://www.spreadfirefox.com/

for more about free software
 http://www.gnu.org

and don't laugh - I know suse contains a lot of non-free software.