Rated 5 out of 5 stars

This Addon is here for historical purpose and reference only.
The subfolder filter author has put comments up that discuss the "applyIncomingFilters" property.
As the author clear stated on their website (see home page URL above, or copy and paste http://www.stroppykitten.com/filterSubFolders/ )

EOL

EOL means End Of Life, no longer supported.

Please read the update.
(Don't use this filter on TBird 4 or above, it'll keep going back to Yes for any folder.)

Thanks Craig for the addon, it was a great one.