Apply this patch to node_modules/selenium-webdriver/firefox/extension.js

extension_diff.patch

--- node_modules/selenium-webdriver/firefox/extension.js	2019-04-23 17:15:51.000000000 -0400
+++ extension_new.js	2020-02-14 13:27:28.221885022 -0500
@@ -176,6 +176,14 @@
     return {id: applications.gecko.id, name, version, unpack: false};
   }
 
+  function setApplication(parsedJSON) {
+    let { browser_specific_settings } = parsedJSON;
+    if (browser_specific_settings && browser_specific_settings.gecko) {
+      parsedJSON.applications = browser_specific_settings;
+    }
+    return parsedJSON;
+  }
+
   function parseXpiFile(filePath) {
     return zip.load(filePath).then(archive => {
       if (archive.has('install.rdf')) {
@@ -186,6 +194,7 @@
       if (archive.has('manifest.json')) {
         return archive.getFile('manifest.json')
             .then(buf => JSON.parse(buf.toString('utf8')))
+            .then(setApplication)
             .then(parseManifestJson);
       }
 
@@ -208,6 +217,7 @@
                 if (jsonExists) {
                   return io.read(jsonPath)
                       .then(buf => JSON.parse(buf.toString('utf8')))
+                      .then(setApplication)
                       .then(parseManifestJson);
                 }
                 throw new AddonFormatError(