diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index e8828859076c..374291ee98a0 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -9504,6 +9504,13 @@ bool nsDocShell::CanLoadInParentProcess(nsIURI* aURI) {
        uri->SchemeIs("moz-extension"))) {
     return true;
   }
+#ifdef MOZ_THUNDERBIRD
+  if (uri->SchemeIs("imap") || uri->SchemeIs("mailbox") ||
+      uri->SchemeIs("news") || uri->SchemeIs("nntp") ||
+      uri->SchemeIs("snews")) {
+    return true;
+  }
+#endif
   nsAutoCString scheme;
   uri->GetScheme(scheme);
   // Allow ext+foo URIs (extension-registered custom protocols). See
diff --git a/toolkit/modules/E10SUtils.jsm b/toolkit/modules/E10SUtils.jsm
index fc2cbf8e9da4..20dfee5f29d3 100644
--- a/toolkit/modules/E10SUtils.jsm
+++ b/toolkit/modules/E10SUtils.jsm
@@ -511,6 +511,13 @@ var E10SUtils = {
 
         return NOT_REMOTE;
 
+      case "imap":
+      case "mailbox":
+      case "news":
+      case "nntp":
+      case "snews":
+        return NOT_REMOTE;
+
       default:
         // WebExtensions may set up protocol handlers for protocol names
         // beginning with ext+.  These may redirect to http(s) pages or to