In Mozilla (a kick-ass browser, may I add), go to the EDIT menu , then select PREFERENCES (at bottom of menu).
In the PREFERENCES pop-up window, click on the PRIVACY list entry (left side), then select the IMAGES sub-entry. Looking at the Image options (right side), make sure the Image Acceptance Policy (top box) is set to ACCEPT ALL IMAGES. So long as you are in there, make sure the bottom ANIMATED IMAGES LOOP section is set to AS MANY TIMES AS IMAGE SPECIFIES.
If this doesn't fix it, make sure you are browsing in this forum, then go to the TOOLS menu item, then select IMAGE MANAGER menu item, then select ALLOW IMAGES FROM THIS SITE.
One of these things should solve the problem. If not, you may be running some other software (firewall, pop-up blocker or ad-blocker) which may be inadvertantly blocking the images.