Bug 1689878 - typo fix and rewording

Merge branch 'staging'

src/releases/seamonkey2.53.7/index.en.html

@@ -282,10 +282,10 @@ Please read this before reporting any new bugs.
 
   <li>Lightning or one of the distributed extensions is disabled under Add-ons.
     <br> Starting with SeaMonkey 2.53.7b1 the distributed extensions can no
-    longer be removed. If you have previously enabled it then under some
+    longer be removed. If you have previously enabled one then under some
     circumstance it might be disabled during the upgrade process. If the
-    extension is disabled and can not be enabled then an old or incomaptible
-    version is still present in your profile.
+    extension is disabled and can not be enabled then it is because an old or
+    incompatible version is still present in your profile.
     <br> To clean out old versions use the following instructions:
     <ul>
       <li>Enter "about:config" in the location bar.</li>