@Rod Steel
If you insist on installing MTube:
1. Load (in NM28) its main installation page:
https://realityripple.com/Software/Mozilla-Extensions/muTube/
2. Place your cersor on the blue "Install MTube v1.2" button
3. Right-click, select "Save Link as"...
4. You should now have on disk file "{35FF1267-2C7D-5FA8-876D-4EDFC0CB89FB}.xpi";
the string is the extension's ID; for brevity, rename file to "MTube-v1.2-pm.xpi"
5. Extract; delete directory "META-INF"
6. In a proper text editor, open file "install.rdf" and browse to the "Pale Moon" block:
<!-- Pale Moon -->
<em:targetApplication>
<Description>
<em:id>{8de7fcbb-c55c-4fbe-bfc5-fc555c87dbc4}</em:id>
<em:minVersion>28.*</em:minVersion>
7. Change the value of minVersion from 28.* to 28.10.4a1, then save your changes
8. Rezip (e.g. with WinRAR) the contents of your working directory [MTube-v1.2-pm] to file "MTube-v1.2-nm28.xpi"
9. Open about:addons in NM28, drag and drop file "MTube-v1.2-nm28.xpi"
10. You should now get a prompt (pop-up window) to install...
Disclaimer: Haven't tested whether the extension itself works as planned post install, especially considering you're running a NM28 version from last August; YMMV...