This isn't actually related to a firefox plugin, or any plugin at all.
crossdomain.xml is part of the flash/flex specification. It's a method to allow/validate cross domain operations for flash and other Adobe products, Sliverlight also seems to use/obey the same adobe policy framework.
From Adobe's Cross-domain policy file specification
A cross-domain policy file is an XML document that grants a web
client—such as Adobe Flash Player, Adobe Reader, etc.—permission to
handle data across multiple domains. When a client hosts content from
a particular source domain and that content makes requests directed
towards a domain other than its own, the remote domain would need to
host a cross-domain policy file that grants access to the source
domain, allowing the client to continue with the transaction. Policy
files grant read access to data, permit a client to include custom
headers in cross-domain requests, and are also used with sockets to
grant permissions for socket-based connections.
Example of a valid crossdomain.xml
<?xml version="1.0" ?>
<allow-http-request-headers-from domain="*" headers="*"/>
So it would seem an Adobe product is asking for permission to perform a cross domain operation - I've seen this in my logs before and took it to be a bot looking for a weakness because I could see no other reason for it being there - I would ignore it if it's doing no apparent harm.