Note: Did not want to post to https://e621.net/forum/show/263380 because it's not quite the same issue. This is not because of slowdowns, etc. It seems directly related to how the browser is interpreting the markup?
Earlier this week, it was working fine, then suddenly no images display unless I click on the Download button. No amount of reloading or such seems to fix it. I've copied and pasted relevant HTML code for the image placeholder that doesn't show up anymore.
Firefox(works):
<div>
<div id="note-container">
</div>
<a name="image"></a>
<img alt="bunch of tags" data-height="800" data-orig_height="1200" data-orig_width="1200" data-resize="true" data-resize_mode="2" data-sample_url="https://static1.e621.net/data/sample/XX/YY/MD5HASH.jpg" data-sample_width="800" data-width="800" id="image" onclick="Note.toggle();" src="https://static1.e621.net/data/sample/XX/YY/MD5HASH.jpg">
</div>
SeaMonkey:
<div>
<div id="note-container">
</div>
<a name="image"></a>
<img alt="bunch of tags" data-height="1900" data-orig_height="1900" data-orig_width="2700" data-resize="true" data-resize_mode="1" data-sample_url="https://static1.e621.net/data/sample/XX/YY/MD5HASH.jpg" data-sample_width="2700" data-width="2700" id="image" onclick="Note.toggle();" src="https://static1.e621.net/data/XX/YY/MD5HASH.jpg" style="width: 718px;">
</div>
The only noticeable difference I noted was that SeaMonkey is using a different link (non-sampled) to the image. It's acting like this div is entirely invisible. No placeholders, so I can't even click on thumbnails in image search results. It literally went from working to not working when I clicked back. I'll try to debug this further and provide more information.
:edit: OK, tried something just be sure. I had whitelisted/disabled all the filters I could, changed user agent to Firefox, etc. I totally disabled ALL plugins, and restarted... still not working. It's possibly something that just BARELY works in Firefox and might fail in future, or it's possibly a bug that got fixed in FF and hasn't been backported yet. My next step is to try to find out if there's a newer version/nightly build that fixes it.
Updated