diff -Naur firefox-52.9.0esr.orig/toolkit/mozapps/installer/packager.py firefox-52.9.0esr/toolkit/mozapps/installer/packager.py
--- firefox-52.9.0esr.orig/toolkit/mozapps/installer/packager.py	2024-01-07 16:54:43.185021639 +0100
+++ firefox-52.9.0esr/toolkit/mozapps/installer/packager.py	2024-01-07 21:33:32.564305725 +0100
@@ -163,17 +163,20 @@
                                   % resource],
                            extra_linker_path=gre_path,
                            extra_env=extra_env):
-            errors.fatal('Error while running startup cache precompilation')
-            return
-        from mozpack.mozjar import JarReader
-        jar = JarReader(cache)
-        resource = '/resource/%s/' % resource
-        for f in jar:
-            if resource in f.filename:
-                path = f.filename[f.filename.index(resource) + len(resource):]
-                if registry.contains(path):
-                    registry.add(f.filename, GeneratedFile(f.read()))
-        jar.close()
+            # errors.fatal('Error while running startup cache precompilation')
+            # return
+            # Ignore the xpcshell errors as it's buggy ATM
+            pass
+        if os.path.exists(cache):
+            from mozpack.mozjar import JarReader
+            jar = JarReader(cache)
+            resource = '/resource/%s/' % resource
+            for f in jar:
+                if resource in f.filename:
+                    path = f.filename[f.filename.index(resource) + len(resource):]
+                    if registry.contains(path):
+                        registry.add(f.filename, GeneratedFile(f.read()))
+            jar.close()
     finally:
         if os.path.exists(cache):
             os.remove(cache)