Can't view Firefox graphics

This is probably a problem with my Mac rather than Firefox, but I can't 
view about 1/4 of the graphics that are on web pages in Firefox. This 
just started happening within the past 48 hours.

I can see the graphics on web pages if I switch to Safari. I can see the 
graphis in Firefox, too, if I hold down my mouse over the missing photo 
- if I choose "View Image" then I go to a web page that's holding the 
image. But the image on the original page is blank.

I've got an iBook G4, and the latest beta version of Firefox, which I 
just installed, and an earlier version, with the same problem, running 
OS X 10.3.9.

Any ideas? Do I need to trash a preference file, or...?

Thanks.