ShipScript has been an eBay Community volunteer since 2003, specializing in HTML, CSS, Scripts, Photos, Active Content, Technical Solutions, and online Seller Tools.
You are correct in nailing the nested font tags problem. We have seen many instances of this. When I put item# 202093635983 in the Sandbox tool, and view with Firefox, the same disappearing act is duplicated in the iframe in that tool.
http://www.isdntek.com/ebaytools/ActiveContentSandbox.htm?202093635983
In fact, I can take the description out of the iframe and view it as a standalone page, and it still won't render in Firefox.
http://vi.vipr.ebaydesc.com/ws/eBayISAPI.dll?ViewItemDescV4&t=1509836431489&item=202093635983
More and more pages are failing to display in my Firefox, and it is due to eBay's editor adding a new font wrapper with each edit, preview, relist, or sell similar.
The Sandbox tool counted 360 font tag wrappers in that listing.
Amazingly, eBay was able to extract 200 characters of text as the mobile summary.
So far looks like everything (tested) newer than Firefox 3.6.24 chokes on it then (I tried FF3.6.28, but don't have any of the pre FF4 alpha minefield versions anymore)
I'm well aware of the stupid eBay inserted font tags as I never use the RTF editor and do everything in the HTML tab - it's become automatic to delete the tags first thing every time I revise or sell similar (thinking very hard about creating a tool to just do that automatically since eBay doesn't seem to be interested in fixing that bug (yeah eBay - it's not a feature).
I only see 75 of the tags in the iframe in Firebug, with 74 of them incomplete, and 363 in a raw dump of the iframe directly with 362 incomplete. They are all closed, but all after the first are missing the size attribute.
I see now why I said the description wasn't visible in the code in Firebug previously - either Firebug or Firefox is choking and quits displaying after the 75th nesting so the remaining 288 or so nested font tags and the nested description text just aren't there.
I don't know if this is a Firefox or an eBay issue. On the one hand Firefox should be displaying valid HTML, but there may be an accepted standard for nesting levels that eBay has violated here that breaks Firefox, and Chrome and other browser happen to handle for now (but might? break at 300, or 500, ...)
Poked around a bit and results inconclusive, but a Stackoverflow thread looking into this with testing by users indicated mixed results. FF3.6 looked to handle upwards off 200k deep total document depth nesting, Webkit had a published spec of 512 deep by default, IE, Chrome, Safari, mixed results depending on element type. Some as rendering glitches, some no display, some plain browser crashes. https://stackoverflow.com/questions/7770573/what-is-the-maximum-depth-of-html-documents-in-practice
So there doesn't appear a set standard, but various browsers handle deep nesting differently with some with set limits, some just crash for memory reasons, etc.
Here's a chunk of very simple test code I modified very slightly from a 13 year old Firefox bug report mentioned in the Stackoverflow thread. It just creates some deeply nested tables with some annotations. That html can be copied to a text file, named as an html file like test-nest.html, and opened in a browser to see results. The failed/not failed are hardcoded and some may no longer be correct after 13 years (the Firefox ones are at least for v31 and 3.6.24), but if an annotation doesn't appear in the middle of the table block then the browser blew chunks and failed to render the html that deep)
<html>
<head>
<style type="text/css"><!--
table
{
border: solid 1px red;
}
--></style>
</head>
<body>
<script type="text/javascript"><!--
function nested( depth, msg )
{
for (var i = 0; i < depth; ++i)
document.write( '<table><tr><td>' );
document.write( depth + ' Nested Tables: ' + msg );
for (var i = 1; i < depth; ++i)
document.write('</tr></td></table>');
document.write('Inside '+depth+': '+msg+'</tr></td></table>Outside '+depth+': '+msg+'<br><br>' );
}
nested( 27, "Works in IE" );
nested( 28, "Fails in IE" );
nested( 33, "Works in Mozilla" );
nested( 34, "Gracefully fails in Mozilla" );
nested( 50, "Gracefully fails in Mozilla" );
nested( 51, "Fails in Mozilla" );
nested( 250, "Fails in Mozilla" );
nested( 500, "Fails in Mozilla" );
nested( 5000, "Fails in Mozilla" );
//--></script>
</body>
</html>
Results in both FF31 and FF3.6.24 (appears that although FF3.6.24 can handle the 350+ nested font tags in the eBay description just fine, it blows up on nested tables)
I'd be inclined to point the finger at eBay on this one. This is something they could fix (just insert the stupid tag once, or add cleanup code to remove extra instances otherwise).
I think the font tag issue was in the back of my mind from your investigations in another thread, and when I chased down that long tree in Firebug looking for the description text it clicked. 
See my previous long response to pocomocomputing - this is going to become more of an issue if eBay doesn't fix it, as the chain of useless font tags grows with relists and sell similars it could start breaking other browsers too.
Additionally, it muddies the http/https View Full Description Button issue waters with an unrelated reason descriptions are not visible.
A simple GreaseMonkey script could automatically strip the "font rwr" tags out every time the listing editor is opened, but 1) it would be non-trivial to parse all the html to ensure that only matched font tag pairs are removed and 2) would not help 99.9% of others who are GreaseMonkey averse, don't use Firefox, or use newest versions of Firefox where GM no longer works or will soon no longer work. The Replace-o-matic Autohotkey script could be used manually as is , or with a modification hardcoded to be manually invoked by the user to do that task, or taking it a step further, made into a daemon to strip the tags.
Best thing would be for eBay to just fix this.
The Bulk Active Content Editor and the Sandbox tool both have a checkbox to remove those extraneous eBay font wrappers, as shown implemented in this Sandbox example where the Markup tab color-codes all those elements.
But the tools only work when the <font rwr> tags are first in the code and the matching </font> tags are last in the code since the tools do not parse elements. If the seller adds an eBay fixit tag to the front, or a promo widget to the end, the tools won't remove the locked-in eBay font tags.
For sellers who do not use font tags at all because they use embedded styles, a regex filter can remove all font tags, regardless of where they are located and regardless of whether they are eBay's, as illustrated in this Sandbox example. On the markup tab we can see that the 800 font tags are all over the place.
I think this growing font issue on eBay's new listing form might be something that brian@ebay could take back to the programmers. I'm seeing more and more disappearing listings as sellers continue to relist or sell similar. And Brian, we've all been wondering what rwr="1" stands for in those eBay tags.
