Detect when browser.contextualIdentities.onRemoved is not supported

For #372

lib/persistentRndStorage.js

@@ -59,11 +59,18 @@
 			settings.persistentRndStorage = newValue? JSON.stringify(scope.persistentRnd): "";
 		});
 		
-		logging.message("register contextual identities removal");
-		browser.contextualIdentities.onRemoved.addListener(function(details){
-			logging.message("Contextual identity", details.contextualIdentity.cookieStoreId, "removed.");
-			clearContainerData(details.contextualIdentity.cookieStoreId);
-		});
+		if (browser.contextualIdentities && browser.contextualIdentities.onRemoved){
+			logging.message("register contextual identities removal");
+			browser.contextualIdentities.onRemoved.addListener(function(details){
+				logging.message("Contextual identity", details.contextualIdentity.cookieStoreId, "removed.");
+				clearContainerData(details.contextualIdentity.cookieStoreId);
+			});
+		}
+		else {
+			logging.error(
+				"Old Firefox does not support browser.contextualIdentities.onRemoved"
+			);
+		}
 	};
 
 	const getInterval = function(){

releaseNotes.txt

@@ -6,7 +6,7 @@ Version 0.5.12:
 		- 
 	
 	fixes:
-		- 
+		- detect when browser.contextualIdentities.onRemoved is not supported
 	
 	known issues:
 		- if a data URL is blocked the page action button does not appear

versions/updates.json

@@ -29,6 +29,14 @@
 				{
 					"version": "0.5.12Alpha20190623",
 					"update_link": "https://canvasblocker.kkapsner.de/versions/canvasblocker_beta-0.5.12Alpha20190623-an+fx.xpi"
+				},
+				{
+					"version": "0.5.12Alpha20190624",
+					"update_link": "https://canvasblocker.kkapsner.de/versions/canvasblocker_beta-0.5.12Alpha20190624-an+fx.xpi"
+				},
+				{
+					"version": "0.5.12Alpha20190625",
+					"update_link": "https://canvasblocker.kkapsner.de/versions/canvasblocker_beta-0.5.12Alpha20190625-an+fx.xpi"
 				}
 			]
 		}