:qa closes the current window (because Vim habits are hard to break and shouldn't be destructive). :exit quits the app.

common/content/dactyl.js

@@ -1839,6 +1839,15 @@ var Dactyl = Module("dactyl", XPCOM(Ci.nsISupportsWeakReference, ModuleBase), {
                 literal: 0
             });
 
+        commands.add(["exit", "x"],
+            "Quit " + config.appName,
+            function (args) {
+                dactyl.quit(false, args.bang);
+            }, {
+                argCount: "0",
+                bang: true
+            });
+
         commands.add(["q[uit]"],
             dactyl.has("tabs") ? "Quit current tab" : "Quit application",
             function (args) {

common/content/tabs.js

@@ -747,8 +747,9 @@ var Tabs = Module("tabs", {
                 });
 
             commands.add(["quita[ll]", "qa[ll]"],
-                "Quit " + config.appName,
-                function (args) { dactyl.quit(false, args.bang); }, {
+                "Quit this " + config.appName + " window",
+                function (args) { window.close(); },
+                {
                     argCount: "0",
                     bang: true
                 });

common/locale/en-US/browsing.xml

@@ -449,6 +449,18 @@ want to bypass &dactyl.appName;'s key handling and pass keys directly to
 
 <h2 tag="quitting save-session">Quitting</h2>
 
+<item>
+    <tags>:x :exit</tags>
+    <strut/>
+    <spec>:exit</spec>
+    <description>
+        <p>
+            Quit &dactyl.appName;, no matter how many tabs/windows
+            are open. The session is not stored.
+        </p>
+    </description>
+</item>
+
 <item>
     <tags>:q :quit</tags>
     <strut/>
@@ -467,8 +479,8 @@ want to bypass &dactyl.appName;'s key handling and pass keys directly to
     <spec>:quita<oa>ll</oa></spec>
     <description>
         <p>
-            Quit &dactyl.appName;, no matter how many tabs/windows
-            are open. The session is not stored.
+            Close the current &dactyl.appName; window, no matter how
+            many tabs are open.
         </p>
     </description>
 </item>