I would run Disk utility and repair permissions (long process). Then I would run Onyx, or Cocktail, or Socks, whatever you have (Onyx is free; make sure you download the version for Leopard). You will need a restart after cleaning caches. Then visit the problematic sites. Might go away. If not, uninstall Firefox and reinstall. Does this happen on all browsers or just Firefox. If not, I would switch to another browser; there is no special reason to use only Firefox. I like Flock, which is close to Netscape.
Tell us how it goes.
I need help with Firefox. Every so often when using Firefox, the screen goes grey and a message appears telling me I need to restart the computer. Most of the time this seems to be caused by one or two certain sites, but it has occasionally happened when Firefox is hidden in the background and not being used. I'm currently using Firefox version 3.5.3 on a new 24-in Intel iMac running OS X 10.5.8.