Add instructions for removing any existing security exception in Firefox (#371)

site/source/user-manual/connecting/connecting-lan/lan-browser/lan-ff.rst

@@ -14,7 +14,7 @@ This should work equally well in Firefox, Firefox ESR, or Librewolf.
 
     .. group-tab:: Linux/Mac/Windows
 
-        #. Open Firefox and enter ``about:config`` in the URL bar. Accept any warnings that may appear about accessing advanced settings.
+        #. Open Firefox and enter ``about:config`` in the URL bar. Accept any warnings that may appear about changing advanced configuration preferences.
 
         #. Search for *security.enterprise_roots.enabled* and double click on *false* so that it turns to *true*:
 
@@ -28,6 +28,20 @@ This should work equally well in Firefox, Firefox ESR, or Librewolf.
                 :width: 80%
                 :alt: Firefox security settings
         
+        .. note:: If you see an exclamation point inside a triangle by the lock, you have made a security exception in the browser.  To remove it, click the lock and go to "Connection not secure":
+
+            .. figure:: /_static/images/ssl/browser/cert-trust-exception-remove-1.png
+                :width: 80%
+                :alt: Firefox - Remove security exception (Part 1)
+
+            Then click "Remove Exception":
+
+            .. figure:: /_static/images/ssl/browser/cert-trust-exception-remove-2.png
+                :width: 80%
+                :alt: Firefox - Remove security exception (Part 2)
+
+            You should now see that the website is trusted as in the final step show above.
+
     .. group-tab:: Android
 
         #. To setup in Firefox Beta or Fennec, tap the kebab menu, then go to *Settings > About Firefox Beta* and tap the Firefox logo five times until it says "Debug menu enabled."