Added /etc/firefox to writable paths to support firefox policies

[kenvandine]

This allows landscape to manage firefox policy profiles

[robert-ancell]

It does raise the question - how many applications require configuration in /etc? It doesn't seem scalable to keep adding lines for every application. Will Firefox support some alternate location for this / is there another scalable way to do this without making the paths writable?

If we migrate to a desktop built on a standard Ubuntu Core this is going to be harder to justify for the core22 snap.

I think for practical reasons given Firefox is such a significant app this is good to land now.

[robert-ancell]

@jhenstridge thoughts on the above?

[jhenstridge]

I think it is unlikely we'll be able to get this change into regular core24 or core26.

I don't have a problem with the change on the understanding that it is an interim measure and a different solution will be needed under the new architecture.