It would be really good to support network links(nfs://user@host/path, smb://user@host/path for example) in the save and open dialog on linux. I can't remember if windows supports this but something like \\host\path should be supported too.
Also have the ability to save those kind of path as bookmarks, ideally by manual typing or as drag and dropping a link/shortcut file as a workaround with the current way the dialog window works.
It's been driving me insane having no way to save to a network folder within firefox without being forced to mount a network path first, which is something I do a lot usually.