Using a global pdfjsWorker property seems problematic here, since we're using the existence of globalThis.pdfjsWorker as an indication that fake-workers are being used.

Correct. What do you think about pdfjsParserWorker ?

Using a global pdfjsWorker property seems problematic here, since we're using the existence of globalThis.pdfjsWorker as an indication that fake-workers are being used.

Correct. What do you think about pdfjsParserWorker ?

Naming things is hard, but maybe pdfjsPreloadedWorker to try and explain "more" what this is?

Another thing that just occurred to me is that we already have the workerPort option, hence rather than "hacking" this into the API it might be possible to just do the following. (Which might also be a tiny bit more efficient, since it avoids some checking during worker-setup.)

diff --git a/web/app_options.js b/web/app_options.js
index 461336dc1..34615b8fc 100644
--- a/web/app_options.js
+++ b/web/app_options.js
@@ -502,7 +502,10 @@ const defaultOptions = {

   workerPort: {
     /** @type {Object} */
-    value: null,
+    value:
+      typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")
+        ? globalThis.pdfjsPreloadedWorker
+        : null,
     kind: OptionKind.WORKER,
   },
   workerSrc: {