diff --git a/lib/main.js b/lib/main.js
index 1604c41..bf7d323 100644
--- a/lib/main.js
+++ b/lib/main.js
@@ -48,6 +48,10 @@
 		message("register port listener");
 		browser.runtime.onConnect.addListener(function(port){
 			notice("got port", port);
+			if (!port.sender.tab){
+				notice("got port without tab = Firefox bug:", port);
+				return;
+			}
 			verbose("send back the tab id", port.sender.tab.id);
 			verbose("send back the persistent random seeds", persistentRndStorage.persistentRnd);
 			port.postMessage({
diff --git a/releaseNotes.txt b/releaseNotes.txt
index 8a9250b..8ebb60d 100644
--- a/releaseNotes.txt
+++ b/releaseNotes.txt
@@ -13,6 +13,7 @@ Version 0.5.4:
 	fixes:
 		- window and audio API were always blocked when using any of the "block ..." modes
 		- canvas content was not stored to be displayed when wanted
+		- hovering over toolbar icon created error in the browser console
 	
 	known issues:
 		- if a data URL is blocked the page action button does not appear