This is likely to be a fairly big project. I don't understand all that's required. We need this for FF5, don't need it for for fennec4.

Printing actually does work already in content process, at least on linux. The progress UI needs to be disabled though.

It will probably require a totally different architecture when we do sandboxing, in that case ;-).

Yes, I'm very well aware of that. But as a first step getting printing working in a simple way is IMO ok.

For sandboxed printing, could we implement another version of nsRenderingContext which would serialize the parameters to method calls and send the data to chrome process? Or does the printing/painting use also lower level APIs (thebes)?

Seems like Thebes is used quite a bit.

sandboxing will depend on azure and azure remoting in any case, so I think we can table the sandboxing portion until that is fleshed out. I'd really like to understand if there is anything left that needs to be done here for desktop Firefox unsandboxed, either core-platform work or making the printing UI work as expected.

For the UI we need to be able forward progress information to chrome, or show a content process window for the progress.

We should probably forward the information rather than trying to create a content-process dialog.