Update css-sticky with more correct Firefox bug

[925259](https://bugzilla.mozilla.org/show_bug.cgi?id=925259) was referring to buggy handling of position:sticky with <thead>. This issue has since been fixed, but the functionality disabled until [35168](https://bugzilla.mozilla.org/show_bug.cgi?id=35168) is fixed. The new tracking bug for this (and other table parts) is [975644](https://bugzilla.mozilla.org/show_bug.cgi?id=975644).