People,
Add-on compatibility depends on 2 sides: the browser side and the add-on side. Because Mozilla has been making constant changes in the browser user interface with the rapid release schedule to push "new features", add-on developers are likewise pushed to make their add-ons compatible with the changed interface. This means that for an add-on to be compatible with Pale Moon 15.*, you may have to run a slightly earlier version of the add-on.
In almost all cases, the incompatible later version add-on is only updated to cater to the changed browser interface, and not to change something fundamental in the add-on.
That being said, if going back to Pale Moon 11 fixes your add-on compatibility, then you may be looking at the other side of this story and need to upgrade your add-on.
Long story short: if your add-on is incompatible because it has been updated to work with the latest version of the Firefox code only, check at addons.mozilla.org in the add-on's page. First, at the bottom, click the arrow next to version information if an add-on says "incompatible", then See complete version history:

- Bottom of AMO page for an add-on
Find an older version that, when hovered over, gives you a clickable (green or striped yellow) button to install it.

- old versions page