... OK, I finally caved in and installed the 30-day trial of Internet Download Manager (IDM) on my Vista machine, just to perform some tests...
Latest version of the app is 6.31build1; the 3 available browser integration extensions are :
idmmzcc.xpi => This is a legacy extension, current version is 7.3.110, its install.rdf mentions Firefox versions 1.5 - 26.0 and Pale Moon versions 3 -25
idmmzcc2.xpi => Legacy extension, current version is 6.27.7; install.rdf file mentions Firefox versions 27.0 - 52.x, but does not specify any Pale Moon versions; however, its chrome.manifest file has line
overlay chrome://browser/content/browser.xul chrome://idm-mozilla_cc/content/overlay.xul application={8de7fcbb-c55c-4fbe-bfc5-fc555c87dbc4}
that application ID is for Pale Moon.
idmmzcc3.xpi => Webextension format, current version is 6.31; manifest.json indicates strict_min_version": "53.0a1" (Firefox 53.0+) .
I then tested those extensions with several of the "roytam1" browser forks...
1. New Moon 27.9.1a1 (fork/rebrand of PaleMoon_unstable 27.9.1a1) (32-bit), buildID=20180615115719
v7.3.110 can be installed via the Moon Tester Tool 1.1.6 palemoon extension, the installation is successful with IDM context menu entries being correctly populated, but the extension does not communicate with main IDM app, hence integration fails
v6.27.7 installs successfully on its own (without MTT) and results in a fully working browser integration with New Moon (as it was reported...)
2. New Moon 28.0.0a4 (fork/rebrand of PaleMoon_unstable 28.0.0a4) (32-bit), buildID=20180615233117
v7.3.110 can be installed via the Moon Tester Tool 1.1.6 palemoon extension, the installation is successful with IDM context menu entries being correctly populated, but the extension does not communicate with main IDM app, hence integration fails
v6.27.7 is itself incompatible with NM28, but can be forcibly installed via MTT; sadly, as reported already, the installation is successful, with IDM context menu entries being correctly populated, but the extension does not communicate with main IDM app, hence integration fails
At this stage, I can't tell whether the blame is to be put on the extension or the (alpha state) browser... Perhaps it should be worthwhile to report it to the Moonchild devs (but say you tried it on official PM28) and emphasize IDM integration works with PM27...
3. Serpent 52.9.0 (fork/rebrand of current Basilisk/UXP-T2) (2018-06-18) (32-bit), buildID=20180618002732
v7.3.110 is incompatible with the browser and can't be installed!
v6.27.7 is compatible and installs successfully; results in a fully working browser integration
v6.31 (WE) is incompatible with the browser and can't be installed!
4. Serpent 55.0.0 (fork/rebrand of deprecated Basilisk/UXP-T1, aka Moebius) (2018-06-16) (32-bit), buildID=20180616141358
v7.3.110 and v6.27.7 (legacy XUL format) are both incompatible and won't install!
However, v6.31 (webextension format) is compatible and installs successfully; results in a fully working browser integration
There you go, if you are a paying customer of IDM, I have listed your working browser options as far as IDM integration is concerned...
Serpent 52.9.0, built on the same platform as New Moon 28, has similar (dare I say better) rendering engine to NM28, so your "site" is bound to also load fine on Serpent 52.9.0 (probably on Serpent 55.0.0, too) ... With the Serpent forks, you get IDM integration as an added bonus...