With Flickron display, click on the site identity button (for details on what that is see the site identity button article) and then on More Information. This will open up the page info dialog.

First select the Permissions panel, make sure that "Load Images" is set to allow (selecting Use Default should also work)

Next select the Media panel, then click on the first item in the list. Use the down arrow key to scroll through the list. If any item has the option "Block images from (domain name)" selected, de-select the option.

This should hopefully resolve your issue, but also see Fix problems that cause images to not show.

Some add-ons can also block images, for example if you have AdBlock Plus installed, make sure that you have not accidentally created a filter to block the images.

Thanks for your guidance. On checking in those lines, I found, the respective settings were already as you have specified. I tested & found it has nothing to do with the kB size of the photo. Could it be, some photos have some special peculiarities, that are rejected by Firefox 3.6.12? A sample is http://www.flickr.com/photos/lokenrc/5163498226/, which I am unable to see in Firefox, but visible in IE