Hi Everyone,
I just found this site today and I still have some more looking around to do.

Classic Add-ons Archive are classic because they are not WebExtensions.
Classic Add-ons Archive is at Version:1.1.2.0 Released:04/17/2018

Here are the links...Have a look for yourselves.

https://justoff.github.io/
https://github.com/JustOff/ca-archive
https://github.com/JustOff/ca-archive/releases
https://github.com/JustOff/ca-archive/blob/master/install.rdf
https://github.com/JustOff/ca-archive/blob/master/update.xml
https://github.com/JustOff/ca-archive/releases/download/1.1.2/ca-archive-1.1.2.xpi [40092 KB]

Have fun

Yeah, it's a great tragedy that just when K-Meleon started to be more compatible with Firefox xpi-addons Mozilla is killing them all :-( This august, over 15.000 addons, all lost!!

There's also a thread here:
http://kmeleonbrowser.org/forum/read.php?9,145607

And considering the newest K-Meleon versions use the Palemoons Goanna engine, some talk about this problem in the PM forum:
https://forum.palemoon.org/viewtopic.php?f=44&t=17829

The "Classic Addons Archive" which you linked, is also just a single addon, by JustOff. What not everyone is aware of, this is only a LIST of all xpi-addons, a mirror of Mozillas addons site, but it does not CONTAIN them. This is impossible, it would require several Gigabytes. And even just this list alone is huge, the xpi-file 40MB plus 60MB for the unzipped sqlite database inside.
What a loss for the world, just because the big corporation chiefs can do what they want, just because they can
Some people try to save what they can, but am not sure at all they will succeed. It's just so MUCH:

A few months ago I was already running into the same problem, with the old Opera12 browser: all addons deleted by the company. At that time it was the only browser I knew for (very) old Windows versions, which still gives access to essential sites like github, sourceforge etc. and also has decent css support. The prob is that meanwhile half the web requires advanced httpS encryption (TLS1.2) even for simply reading public webpages, a real killer feature for old systems and browsers.
Old Opera Presto still helps a lot, it has partial support for TLS1.2 and still shows a lot more websites today as FF3.6 or KM1.6, and it was a highly customizable browser in 2012. But do you think I'd have found on the web now ANY extensions anymore? Absolutely NOTHING, all deleted, everywhere! All gone. Not even a modern UserAgent switcher to be found anymore, or any help pages how to update the ancient strings in the included switcher. Also most Help and Howto-pages - all gone. After long, long searching finally found some stuff archived, but mainly blog posts, still no addons.

Mozilla now does the same. Simply deleting ALL addons for older Firefox versions and forks like K-Meleon and Palemoon.

So if you can, better hurry and try to find the ones you possibly want to use in the future with K-Meleon, or with an older Firefox or Palemoon version Although, don't expect them to run fully, those requiring buttons and menus are still a major prob.

Make sure you download not the newest addon versions (NOT type WebExtension), but older versions for Firefox. K-Meleon 74 (gecko) is close to FF24, KM75 to FF31, KM76 to FF38.
KM77-Goanna/Palemoon is floating somewhere between FF38-52, if I got that right (?)

PS: for (very) old systems there's also Firefox9, learned far too late about this one and Opera12.
But currently roytam1 is working on KM74-Goanna, a huge step forward to the web of 2015/16