I had the same thing happening a couple of months ago, also using Firefox. It turns out that was some subtle thing in the way the browser was interacting with the server software. Here's what I did to solve the problem:
I displayed one of the pages where the problem was occurring, then Viewed the Page Source and located the HTML code that was trying to display the image. (The code will contain an ALT tag with the text that you're seeing displayed in place of the image.) I then selected and copied the URL for the image into my browser window, and hit enter.
Here's where I discovered what the problem was. Rather than get the image, I got a message asking me to log in. Rather confusing, since I had been logged in all along, but I went ahead and re-entered my log-in information. The image was then displayed, and I haven't had the problem reoccur since then.
Ken
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Ken Maurer