Anyways, it seems that on Linux, Pale Moon seems to prefer KDE applications to Trinity applications when clicking "show in folder" in the "Downloads" menu, and there's no way to override that preference.
On the first system, I have both Trinity and KDE versions of Dolphin file browser installed (the latter pulled in automatically by APT when I went to install KWin from Plasma 5 (because Trinity's Compiz support is there, but broken)). When I go to the "Downloads" menu and click "show in folder", KDE's Dolphin opens up instead of Trinity's Dolphin.
I would have been able to live with the newer version of Dolphin, had I known (back then) that I could have just installed the Ubuntu 16.10 package for Qt5ct onto my Debian 9 installation, add the necessary file to ~/.trinity/env/, then log back out and log back in, and set Qt5ct to use a theme similar to the TQt theme i already have set*.
I didn't do that for the first system (I don't have access to that system now, for reasons which would be hard for me to explain, but I would need to reinstall Debian on that system anyway), but I did just that for the second system, which I do have access to. However, as I did not need the newer KWin on that system, KDE's Dolphin didn't get pulled in, but I had KDE's Gwenview installed, and PM went and used that as the file manager for its "show in folder" function, instead of Trinity's Dolphin.
Here are all the currently set environment variables on the second system:
I forgot to mention, this bug may be due to it only checking /usr/bin/ instead of all folders in the PATH variable.