I have a very similar problem. Both WP and S:P are on the latest versions and the problem has reared it’s head since the latest S:P update. I am using a fairly old version of Linux Mint with FF 20.0, as it is an old version, nothing has been updated recently. I have also had reports from Members using Windows and the latest version of FF to the same effect.
Just before the update an error started with pictures uploaded directly to the forum in that the sizes were not added to the HTML tag meaning that the picture did not display in the edit box, although when the message was posted it did appear. This resulted in the picture being displayed multiple times as forum users thought the pictures were not added as they couldn’t see them in the edit box (exactly as described in the first 2 posts).
Since the update, that particular error seems to be less of a problem although I am reluctant to say it never occurs, and now a different problem has arisen although strangely similar.
Some Members use Photobucket to host their pictures and have always posted links to them which has always worked fine, now, using the same methods as before, they enter the code supplied by Photobucket and whilst in the edit box the picture appears as normal. As soon as the message is posted, instead of the picture there is a line stating ‘Image cannot be found’! If the edit box is opened, the image is shown as expected. This is both on new posts and earlier posts that have, until recently, been working perfectly.
I followed the instructions suggested by Brandon in this thread and unchecked ‘Use popup image enlargement’ and ‘Always use image thumbnails’ and the images now appear in the messages. Both the above have to be unchecked for this to work.
There is another Topic here https://simple-press.com/support-forum/sp5-general-topics/minor-image-inserting-issue-under-safari-and-google-chrome/ which seems to have some relevance to both the above problems, and another here https://simple-press.com/support-forum/sp5-plugin-topics/tinymce-and-html/
As an example, below is the code one of the Members used from Photobucket which displays the above behaviour.
NOTE:- If I re-check the 2 settings S:P reverts to showing the error message so the error is able to be reproduced.
<p><a href="http://s1125.photobucket.com/user/bsathunderbolt/media/Gilera%20Arcore/D1%20Bantam%20Rear%20Wheel%20Bearings/IMG_0960_zps5f7cdf25.jpg.html"><img alt="http://i1125.photobucket.com/albums/l585/bsathunderbolt/Gilera%20Arcore/D1%20Bantam%20Rear%20Wheel%20Bearings/IMG_0960_zps5f7cdf25.jpg" src="http://i1125.photobucket.com/albums/l585/bsathunderbolt/Gilera%20Arcore/D1%20Bantam%20Rear%20Wheel%20Bearings/IMG_0960_zps5f7cdf25.jpg" /></a></p>