Yes, but not yet. We won't start pushing advertised updates to Firefox 3.x users until a bit after the launch, to give time for any late-breaking issues to shake out.

Mine didn't seem to (Windows 7 Enterprise, 64-bit).

The first thing I did after manually downloading and installing was to re-visit http://www.iquilezles.org/apps/shadertoy/, posted here a few days ago. Amazing.

IT works!! go to help -> check for updates

It updated to 4 for me (on Mac), although it goes in sequential order; e.g. if there's any 3.x you didn't get it seems to update that first, then on relaunch another update-check gets version 4.

win7, I tried update (using the menu) with no luck, but the install process was easy (installed it in my old ff folder and it upgraded everything) and all my settings were kept.

Some of my plug-ins are no longer compatible though...

Just checked, and it doesn't (Mac)

Historically major updates have been offered only after a minor release or two.