[Flashblock] Flashblock and Firefox 1.5

[Philip Chee]

Note to self:

In Flashblock 1.5 branch remove all the code from flashblock.js that
reference userContent.css (read/write/IO) since we are now using
nsIStylesheetService.

Although Seamonkey is based on Gecko 1.8, it doesn't use the
install.manifest file, therefore it needs the contents.rdf files.
Hence: remove Seamonkey support from Flashblock 1.5.
We will support Seamonkey via the 1.3 branch.