diff --git a/thunderbirdparser/src/org/sleuthkit/autopsy/thunderbirdparser/ThunderbirdMboxFileIngestModule.java b/thunderbirdparser/src/org/sleuthkit/autopsy/thunderbirdparser/ThunderbirdMboxFileIngestModule.java
index 0000847c63..13ca786c3b 100644
--- a/thunderbirdparser/src/org/sleuthkit/autopsy/thunderbirdparser/ThunderbirdMboxFileIngestModule.java
+++ b/thunderbirdparser/src/org/sleuthkit/autopsy/thunderbirdparser/ThunderbirdMboxFileIngestModule.java
@@ -31,6 +31,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 import java.util.logging.Level;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -84,9 +85,10 @@ public final class ThunderbirdMboxFileIngestModule implements FileIngestModule {
     private Blackboard blackboard;
     private CommunicationArtifactsHelper communicationArtifactsHelper;
     
-    // A cache of custom attributes for the VcardParser unique to each ingest run.
-    private Map<String, BlackboardAttribute.Type> customAttributeCache;
-
+    // A cache of custom attributes for the VcardParser unique to each ingest run, but consistent across threads.
+    private static ConcurrentMap<String, BlackboardAttribute.Type> customAttributeCache = new ConcurrentHashMap<>();
+    private static Object customAttributeCacheLock = new Object();
+    
     private static final int MBOX_SIZE_TO_SPLIT = 1048576000;
     private Case currentCase;
 
@@ -100,7 +102,13 @@ public final class ThunderbirdMboxFileIngestModule implements FileIngestModule {
     @Messages({"ThunderbirdMboxFileIngestModule.noOpenCase.errMsg=Exception while getting open case."})
     public void startUp(IngestJobContext context) throws IngestModuleException {
         this.context = context;
-        this.customAttributeCache = new ConcurrentHashMap<>();
+        
+        synchronized(customAttributeCacheLock) {
+            if (!customAttributeCache.isEmpty()) {
+                customAttributeCache.clear();
+            }
+        }
+        
         try {
             currentCase = Case.getCurrentCaseThrows();
             fileManager = Case.getCurrentCaseThrows().getServices().getFileManager();
@@ -917,7 +925,11 @@ public final class ThunderbirdMboxFileIngestModule implements FileIngestModule {
 
     @Override
     public void shutDown() {
-        // nothing to shut down
+        synchronized(customAttributeCacheLock) {
+            if (!customAttributeCache.isEmpty()) {
+                customAttributeCache.clear();
+            }
+        }
     }
 
 }
diff --git a/thunderbirdparser/src/org/sleuthkit/autopsy/thunderbirdparser/VcardParser.java b/thunderbirdparser/src/org/sleuthkit/autopsy/thunderbirdparser/VcardParser.java
index 6e3ff0082b..306843f025 100755
--- a/thunderbirdparser/src/org/sleuthkit/autopsy/thunderbirdparser/VcardParser.java
+++ b/thunderbirdparser/src/org/sleuthkit/autopsy/thunderbirdparser/VcardParser.java
@@ -40,6 +40,7 @@ import java.util.Collection;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ConcurrentMap;
 import java.util.logging.Level;
 import org.apache.commons.lang3.StringUtils;
 import org.openide.util.NbBundle;
@@ -102,12 +103,12 @@ final class VcardParser {
      * A custom attribute cache provided to every VcardParser from the
      * ThunderbirdMboxFileIngestModule, but unique to one ingest run.
      */
-    private final Map<String, BlackboardAttribute.Type> customAttributeCache;
+    private final ConcurrentMap<String, BlackboardAttribute.Type> customAttributeCache;
     
     /**
      * Create a VcardParser object.
      */
-    VcardParser(Case currentCase, IngestJobContext context, Map<String, BlackboardAttribute.Type> customAttributeCache) {
+    VcardParser(Case currentCase, IngestJobContext context, ConcurrentMap<String, BlackboardAttribute.Type> customAttributeCache) {
         this.context = context;
         this.currentCase = currentCase;
         tskCase = currentCase.getSleuthkitCase();