Also check for config file, not just the registry

packages/thunderbird-config.xml

@@ -8,13 +8,16 @@
 
         <depends package-id="thunderbird" />
 
-        <variable name='PKG_VERSION' value='6' />
+        <variable name='PKG_VERSION' value='7' />
         <variable architecture='x86' name='PROG' value='%PROGRAMFILES%\Mozilla Thunderbird' />
         <variable architecture='x64' name='PROG' value='%PROGRAMFILES(x86)%\Mozilla Thunderbird' />
         <variable architecture='x86' name='REG_PATH' value='HKLM\Software\Mozilla\Thunderbird' />
         <variable architecture='x64' name='REG_PATH' value='HKLM\Software\Wow6432Node\Mozilla\Thunderbird' />
 
         <check type="registry" condition="equals" path="%REG_PATH%\ConfigRevision" value="%PKG_VERSION%" />
+        <check type="file" condition="exists" path="%PROG%\defaults\pref\autoconf.js" />
+        <check type="file" condition="exists" path="%PROG%\thunderbird.cfg" />
+        <check type="file" condition="exists" path="%PROG%\chrome\custom-strings.txt" />
 
         <install cmd='%COMSPEC% /C copy /Y "%SOFTWARE%\thunderbird\autoconf.js" "%PROG%\defaults\pref\"' />
         <install cmd='%COMSPEC% /C copy /Y "%SETTINGS%\thunderbird\thunderbird.cfg" "%PROG%\"' />