Hi

This is currently not possible in Firefox, but it is something that has been requested by quite a few people in our feedback site at https://connect.mozilla.org/ and is something that our developers are looking into.

I guess I will hear from Firefox when it is possible to upload and set a background image.

Fingers crossed it is something that can be included very soon (it is something that I would like as well!).

In the meantime, have you seen this extension?

Mue https://addons.mozilla.org/en-US/firefox/addon/mue/