This is an attempt to keep track of features that are very commonly requested by extension/application developers that the platform is currently lacking.
Please do not just add your favourite feature to this list, it will be added to as features are clearly widely needed by watching requests in the development channels and newsgroups.
nsIProcess is pretty poor for running external programs.
There is currently no easy way to parse some file/string as html without using some kind of hidden iframe or other hacks.
nsIProcess fails to support some of the its own interface methods. It would also be nice to know if the process is still running, for times when you start a process async.
Creating an image without using canvas
The are situations when embedding or using C++ components where creating an image from an element or document is desired, but very difficult. The developers rarely want to use a cairo or thebes surface. They just want to pass a stream or filename along with an image mime type.
Access from external applications
External apps (e.g. installers) sometimes wish to modify the state of the app, e.g. cookies, proxy settings, bookmarks. Some also wish to find things like the currently loaded URL. (I've told them to use the accessibility stuff to get there).