I've just tested this and you're right. I installed the latest version of the “Seamonkey” web browser - version 2.53.21 available from https://www.seamonkey-project.org/releases/ - in a VM (Virtual Machine) running "Ubuntu MATE 24.04.3 LTS" and then I used Seamonkey to access this "Ubuntu MATE Community" web site - https://ubuntu-mate.community/ - and I also got the same result that you had.
Unfortunately, I don't believe there's much that can be done about it. I say this because this "Ubuntu MATE Community" uses / is based on the open-source forum software called "Discourse" - https://www.discourse.org/ - and "Discourse”'s support policy, regarding web browsers, is the following:
” (...) What are the minimum browser requirements?
Discourse is designed for the next 10 years of the Internet, so the minimum browser requirements are high.
Discourse supports the latest, stable releases of all major browsers and platforms:
Microsoft Edge
Google Chrome
Mozilla Firefox
Apple Safari (including Safari on iOS 16.4+)
(…)”
I’ve found the following discussion topic, in the Official forum about “Discourse” itself (that forum is called “Discourse Meta” - http://meta.discourse.org), a topic that is specifically about “Seamonkey” (apparently, this situation exists at least since 2023):
So, I guess that the best advice that I can give is to use one of the four web browsers that “Discourse” officially supports (“Microsoft Edge”, “Google Chrome”, “Mozilla Firefox” or “Apple Safari”) to access this “Ubuntu MATE Community”. Sorry!
I don't know if I am walking a tightrope, in terms of how long I will be able to continue doing this, but when I go from OS Distro/Version to OS Distro/Version, always forward, never retrograde,
I install the new OS with new Firefox,
I start Firefox, to auto-create the "default" profile,
I rename the ~/.mozilla/firefox folder to something different, then
I restore my backup of the ~.mozilla/firefox from the my backup of my previous Distro/OS installation.
I have never had problems so far,
no loss of bookmarks (to be expected), and
no loss of the SQLite fields for Tags and Keywords (tags are no longer supported in the Firefox API )
If I go with the default install from the software, I lose access to those fields, which for me are critical!
I am still convinced that the restriction on installing the earlier 139 has to do with conflict from remnants/leftovers from an incomplete un-install of the later 142 version ... and ... that you are trying to install using a method that replicates the 142 install, which is prone to exposing a 139 re-install to all sorts of package-installer checking tests which should, by rights, have no relevance, if you've taken the proper precautions with backing up your data under ~/.mozilla before you did the 142 un-install, followed by the 139 re-install ... especially if simply dropping in a tarball extract!!!
But, clearly, you've chosen to take the off-ramp to Waterfox, and that's absolutely exercising your freedom of choice!
Ain't Linux wonderful?
BTW, that posting on MozillaZine has too little information to be of any real benefit to anyone attempting to revert to an older version, IMHO. All it does is offer reports by others confirming the problem on 142, which is a different issue and, while I concede that issue is of importance to many, is unrelated to the issue of reverting back to an earlier version.
BTW, if you think there is an issue with losing bookmarks from Firefox, there is a non-Firefox plug-in for selective export/import of bookmarks: