Now that Lego Wedo support has been formally announced:

http://scratch.mit.edu/discuss/topic/36203/

I tested the driver download and it has been updated to a newer version.

Scratch doesn't appear to warn you of this update or even tell you which version plugin you are using, so I thought I'd mention it here for those of us who have been using the older version plugin for a while.

The updated drivers are here:

http://cdn.scratch.mit.edu/scratchr2/static/__1398804290__//ext/ScratchDevicePlugin.msi (windows)
http://cdn.scratch.mit.edu/scratchr2/static/__1398804290__//ext/ScratchDevicePlugin.dmg (mac)

The good news is that on Windows, after installing and restarting your computer Firefox now also works as well as Chrome.
I still couldn't get IE to work though.

Even better news is you can now use 2 motors. No powerfunctions lights or powerfunctions servo support yet though.

The new version was installed here on Windows 7, so appears to be plugin version 1.0.0.0:
C:\Users\Your_User_Name\AppData\Roaming\MITMediaLab\Scratch Device Plugin\1.0.0.0\npScratchDevicePlugin.dll

scratch.mit.edu/projects/10652808/

I cannot get the Lego WeDo plug in to work on Scratch from any browser on Windows 8. IE, FireFox and Chrome all don't work. Any ideas?

Try the unofficial plugin here instead:

www.picaxe.com/s2bot