Sorry for the confusion, Graeme.
The File Tabs tool window is the old way we presented open files. We kept it around because users that do _not_ use one-file-per-window may still find it useful (and maybe more comfortable as they make the transition to the new MDI layout). If you do use one-file-per-window, then the old File Tabs tool window was less than useful since it only displayed 1 Tab per file. Multiple windows viewing the same file were not represented.
When you toggled "Abbreviate similar" you were doing it for the old File Tabs tool window. You can turn it off for the new Tabs from Options>Editing>Editor Windows, Abbreviate file tab captions.