diff --git a/browser/components/shell/ShellService.sys.mjs b/browser/components/shell/ShellService.sys.mjs
index 09f23a51670f3..c4af0be7de229 100644
--- a/browser/components/shell/ShellService.sys.mjs
+++ b/browser/components/shell/ShellService.sys.mjs
@@ -170,6 +170,7 @@ let ShellServiceInternal = {
   getDefaultPDFHandler() {
     const knownBrowserPrefixes = [
       "AppXq0fevzme2pys62n3e0fbqa7peapykr8v", // Edge before Blink, per https://stackoverflow.com/a/32724723.
+      "AppXd4nrz8ff68srnhf9t5a8sbjyar1cr723", // Another pre-Blink Edge identifier. See Bug 1858729.
       "Brave", // For "BraveFile".
       "Chrome", // For "ChromeHTML".
       "Firefox", // For "FirefoxHTML-*" or "FirefoxPDF-*".  Need to take from other installations of Firefox!
diff --git a/toolkit/mozapps/defaultagent/DefaultBrowser.cpp b/toolkit/mozapps/defaultagent/DefaultBrowser.cpp
index 1edfeef300ceb..975af9beb2b1e 100644
--- a/toolkit/mozapps/defaultagent/DefaultBrowser.cpp
+++ b/toolkit/mozapps/defaultagent/DefaultBrowser.cpp
@@ -145,11 +145,16 @@ static BrowserResult GetDefaultBrowser() {
         // "AppX[hash]" as expected. It is unclear if the EdgeWithEdgeHTML and
         // EdgeWithBlink ProgIDs would differ if the latter is changed into a
         // package containing Edge.
-        constexpr std::wstring_view progIdEdgeHtml{
+        constexpr std::wstring_view progIdEdgeHtml1{
             L"AppXq0fevzme2pys62n3e0fbqa7peapykr8v"};
-
-        if (!wcsnicmp(registeredApp.get(), progIdEdgeHtml.data(),
-                      progIdEdgeHtml.length())) {
+        // Apparently there is at least one other ProgID used by EdgeHTML Edge.
+        constexpr std::wstring_view progIdEdgeHtml2{
+            L"AppXd4nrz8ff68srnhf9t5a8sbjyar1cr723"};
+
+        if (!wcsnicmp(registeredApp.get(), progIdEdgeHtml1.data(),
+                      progIdEdgeHtml1.length()) ||
+            !wcsnicmp(registeredApp.get(), progIdEdgeHtml2.data(),
+                      progIdEdgeHtml2.length())) {
           return Browser::EdgeWithEdgeHTML;
         }
       }