The Support sections used to also allow Guest posting until that was stopped (at end of March 2011) due to well Guest spam being too much.

General Questions about Firefox :) .

Support is aimed mostly at how do I do this, how do I fix this or why does this not work.

General is more about why was this added (or removed), discussion about things in Firefox's future and, to an extent, items that are being covered in Support but would be white noise in the actual support thread. You might have noticed that most FAQ stickies in support are locked and have a listed discussion thread in General.

Builds is straight forward. Discussion there is limited to pre-release versions of Firefox.

Features and bugs are not all that important any more. Bugs was meant to be a discussion area either for conversation about adding a bug to bugzilla or discussion about an existing bug. Features was about most non-support discussion of existing features or discussion about wanted features.

What would go in General that wouldn't fit in one of the other categories?

They only get the posts that people initiate in each. Each is for a different purpose. Moderators move the posts that are obviously support related from the General to the Support forum.

The General and Support get the same posts. Why not combine these?