Update: On a second machine running PM 28.2.1, I'm not seeing this bug, so when I get around to updating this machine, I'll report back.
(Not sure why I'm not getting updated here automatically, but will find that out too.)
I'm seeing this a bit more often now, and found a generally accessible website (Washington Post) and article where it happens for me:
I did a 'view-source', and located a specific href which I can access consistently where the problem repeats. Note, I have to start Palemoon, and then enter the above URL in order to see the broken images. Once I click on them (which makes them appear), they remain visible, even if I leave and then return to the same url. So, I infer that means this is a caching problem.
Here's the source which (I believe) brings up one of the broken images...
Berger’s act is a tribute to Al Jolson, the vaudeville superstar of nearly a century ago who had his biggest hit in “<a href="http://www.amazon.com/gp/product/B00005JKSC/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=B00005JKSC&linkCode=as2&tag=thewaspos09-20&linkId=X236HYVIHITEUREP" target="_blank" title="www.amazon.com">The Jazz Singer</a>,”
FYI, I had installed an add-on called 'Greedy Cache', and when I saw this problem the first time, I disabled it.