diff --git a/python/mozbuild/mozbuild/base.py b/python/mozbuild/mozbuild/base.py
index d43d74de9333..5644e955fe21 100644
--- a/python/mozbuild/mozbuild/base.py
+++ b/python/mozbuild/mozbuild/base.py
@@ -186,16 +186,19 @@ class MozbuildObject(ProcessExecutionMixin):
                 topsrcdir, topobjdir, mozconfig = load_mozinfo(mozinfo_path)
                 break
 
-        if not topsrcdir:
-            # See if we're running from a Python virtualenv that's inside an objdir.
-            # sys.prefix would look like "$objdir/_virtualenvs/$virtualenv/".
-            # Note that virtualenv-based objdir detection work for instrumented builds,
-            # because they aren't created in the scoped "instrumentated" objdir.
-            # However, working-directory-ancestor-based objdir resolution should fully
-            # cover that case.
-            mozinfo_path = os.path.join(sys.prefix, "..", "..", "mozinfo.json")
-            if detect_virtualenv_mozinfo and os.path.isfile(mozinfo_path):
-                topsrcdir, topobjdir, mozconfig = load_mozinfo(mozinfo_path)
+            # We choose an arbitrary file as an indicator that this is a
+            # srcdir. We go with ourself because why not!
+            our_path = os.path.join(
+                dir_path, "python", "mozbuild", "mozbuild", "base.py"
+            )
+            if os.path.isfile(our_path):
+                topsrcdir = dir_path
+                break
+
+        # See if we're running from a Python virtualenv that's inside an objdir.
+        mozinfo_path = os.path.join(os.path.dirname(sys.prefix), "../mozinfo.json")
+        if detect_virtualenv_mozinfo and os.path.isfile(mozinfo_path):
+            topsrcdir, topobjdir, mozconfig = load_mozinfo(mozinfo_path)
 
         if not topsrcdir:
             topsrcdir = os.path.abspath(