Add Thunderbird 68 support (legacy mode).

install.rdf

-<?xml version="1.0"?>
-<RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
-     xmlns:em="http://www.mozilla.org/2004/em-rdf#">
-  <Description about="urn:mozilla:install-manifest">
-    <em:id>unsafelinks@ant.sr</em:id>
-    <em:name>Un-Safelinks</em:name>
-    <em:description>Replace URLs which have been rewritten by Office 365 Advanced Threat Protection with the original URL.</em:description>
-    <em:version>1.2</em:version>
-	<em:homepageURL>https://ant.sr/unsafelinks</em:homepageURL>
-    <em:targetApplication>
-      <Description>
-        <em:id>{3550f703-e582-4d05-9a08-453d09bdfdc6}</em:id>
-        <em:minVersion>45.8.0</em:minVersion>
-        <em:maxVersion>60.*</em:maxVersion>
-      </Description>
-    </em:targetApplication>
-  </Description>
-</RDF>

manifest.json

+{
+    "manifest_version": 2,
+    "name": "Un-Safelinks",
+    "description": "Replace URLs which have been rewritten by Office 365 Advanced Threat Protection with the original URL.",
+    "version": "1.3",
+    "author": "Anton Sarukhanov",
+    "homepage_url": "https://github.com/antsar/unsafelinks",
+    "applications": {
+        "gecko": {
+            "id": "unsafelinks@ant.sr",
+            "strict_min_version": "68.0"
+        }
+    },
+    "permissions": [
+        "messagesRead"
+    ],
+    "icons": {
+        "64": "icon.png"
+    },
+    "legacy": {
+        "type": "xul"
+    }
+}