diff --git a/build/clang-plugin/CustomMatchers.h b/build/clang-plugin/CustomMatchers.h
index 5a74c155707b..ece717e79918 100644
--- a/build/clang-plugin/CustomMatchers.h
+++ b/build/clang-plugin/CustomMatchers.h
@@ -194,17 +194,34 @@ AST_MATCHER(CallExpr, isInWhiteListForPrincipalGetUri) {
 /// code or names of existing threads that we would like to ignore.
 AST_MATCHER(CallExpr, isInAllowlistForThreads) {
 
-  // Get the source location of the call
+  // Get the source location of the call.
   SourceLocation Loc = Node.getRParenLoc();
   StringRef FileName =
       getFilename(Finder->getASTContext().getSourceManager(), Loc);
+
+  const auto rbegin = [](StringRef s) { return llvm::sys::path::rbegin(s); };
+  const auto rend = [](StringRef s) { return llvm::sys::path::rend(s); };
+
+  // Files in the allowlist are (definitionally) explicitly permitted to create
+  // new threads.
   for (auto thread_file : allow_thread_files) {
-    if (llvm::sys::path::rbegin(FileName)->equals(thread_file)) {
+    // All the provided path-elements must match.
+    const bool match = [&] {
+      auto it1 = rbegin(FileName), it2 = rbegin(thread_file),
+           end1 = rend(FileName), end2 = rend(thread_file);
+      for (; it2 != end2; ++it1, ++it2) {
+        if (it1 == end1 || !it1->equals(*it2)) {
+          return false;
+        }
+      }
+      return true;
+    }();
+    if (match) {
       return true;
     }
   }
 
-  // Now we get the first arg (the name of the thread) and we check it.
+  // Check the first arg (the name of the thread).
   const StringLiteral *nameArg =
       dyn_cast<StringLiteral>(Node.getArg(0)->IgnoreImplicit());
   if (nameArg) {
diff --git a/build/clang-plugin/ThreadFileAllows.txt b/build/clang-plugin/ThreadFileAllows.txt
index dcdb218ca761..ed51dbd4cfd8 100644
--- a/build/clang-plugin/ThreadFileAllows.txt
+++ b/build/clang-plugin/ThreadFileAllows.txt
@@ -1,18 +1,23 @@
 # This file is ingested by `ThreadAllows.py` to produce a list of files which
 # our clang plugin will allow to use `NS_NewNamedThread`.
 #
-# Note that this file contains a list of _files_, not _paths_.
+# Files may be specified with any number of slash-separated path-elements; all
+# provided path-elements must match. (Because we often move and/or symlink
+# header files, this means headers will usually have no path-elements.)
+#
+# Note that this file contains a list of _files_, not _paths_. The clang plugin
+# has no notion of $TOPSRCDIR.
 
 ######
 # Release files
 
 # declaration and definition of `NS_NewNamedThread`
 nsThreadUtils.h
-nsThreadUtils.cpp
+xpcom/threads/nsThreadUtils.cpp
 
 # Thread-pools are permitted to make dynamically many threads, using dynamic
 # thread names with explicit numbering.
-nsThreadPool.cpp
+xpcom/threads/nsThreadPool.cpp
 
 ######
 # Unsorted files
@@ -20,7 +25,8 @@ nsThreadPool.cpp
 # Files below this point are grandfathered in. Please do not add new files to
 # this list -- and please remove any that you can, whether by documenting and
 # moving them or by confirming that they are no longer required.
-ActorsParent.cpp
+dom/indexedDB/ActorsParent.cpp
+dom/quota/ActorsParent.cpp
 DecodePool.cpp
 GeckoChildProcessHost.cpp
 LazyIdleThread.cpp