Tab Atkins is working on a new draft spec for XUL-style "flexible boxes" in CSS. One issue that has come up is whether the XUL concept of "preferred widths" is essential or not. It would be really useful if XUL developers could contribute to the thread in www-style with feedback on that and other issues in related threads.
More detail: the children of a display:-moz-box element can have both width and -moz-box-flex CSS properties set on them. The width property sets a preferred width for each child. Any leftover space in the container is allocated to the children in proportion to their -moz-box-flex values. Likewise, if the preferred widths add up to more than the width of the container, the excess space is trimmed from the children in proportion to their -moz-box-flex values.
An alternative, simpler model would be to not have preferred widths: just size each child proportionally to its flex value. min-width and max-width could still be honoured. It's unclear whether this simpler model is too simple.
So if you have something to say about how you use flexboxes, now's the time to contribute and ensure your needs are addressed!