How can I get WebStorm to lookup entity references in my DTD files

Question

I'm using JetBrains WebStorm 2023.1, to work on a Thunderbird extension.

My extension includes some XUL/XHTML files whose content gets overlaid on top of Thunderbird. These files contain may attribute values which are placeholders for DTD lookup, e.g.

<label value="&spaceBetweenParagraphs.label;"/>

... and WebStorm complains about these, with the error:

Unresolved entity reference spaceBetweenParagraphs.label

Now, Thunderbird knows where to look for these definitions, because the XML has a reference to it:

  <!ENTITY % bdmDTD SYSTEM
             "chrome://bidimailui/locale/bidimailui-prefs-dialog.dtd">
  %bdmDTD;

... but the URI is not valid except when the extension has been loaded. In the source tree, there is a locale/ directory with subdirectories for different languages.

My question: How can I tell WebStorm to try to look up entity references in a specific DTD file (or several files)?

Note: I wouldn't mind if WebStorm only looked at, say, the en-US locale.