CSS3text-transform implementation in Firefox is under the spotlight since a couple of weeks.
After supporting specific cases for the Dutch IJ digraph (w/ text-transform:capitalize) and the turkic i/İ and ı/I pairings, it is now the case of a specific Greek rule.
In Greek, there are two lower-case sigmas (well-known by mathematicians in its uppercase representation: Σ). Lower-case sigma can be represented by two different signs: σ and ς. The choice of one or the other is based on a simple rule: if the sigma is alone or non-terminal, the σ sign is used. If it is a word-terminal (and not the only character of the word), the ς sign is used.
This is a special case in CSS only in the case where text-transform:lowercase is applied to uppercase greek text. Not very common, but this is fixed in today Nightly.