> I suspect most screenreaders would simply say "Eff Eff Ess".
Correct, thankfully. I don't really see a use case for screen reader-specific usage of the abr tag. If everybody else has to read "FFS", possibly having to look it up on Google to know what on earth it is, why wouldn't I also? Although I suspect that as a sighted user, you can mouse-over some text marked up with abr and see the expanded form.
<abbr title="Fat Finger Syndrome">FFS</abbr>
This also helps reduce ambiguity about the meaning of an acronym, improving the communication of semantic intent.
That said, it appears that expansion of <abbr> isn't consistently supported[1].
I suspect most screenreaders would simply say "Eff Eff Ess".
[1] http://www.powermapper.com/tests/screen-readers/labelling/ac...