There are 2 users at least (Fram and myself)
reporting that the new option does not seem to work when using FF (in our case, version 25).
I tried to VEdit my sandboxes in several languages.
You can use https://it.wikipedia.org/wiki/Utente:Elitre/Sandbox?veaction=edit if you need to.
After you click on the Switch button, and click OK on the warning message, the surfaces is "grayed out", and just stays that way without actually switching.
Please notice you can still VEdit at this time, and even save.
The chosen skin instead does not seem to matter (it didn't work for me with any of them).
In Fram's words:
<< I get the "you are about to change" message, but then the VE page gets "bleached" out, as if an opaque glass is put in front of it, and nothing further happens. I can still use the toolbox at this time, and can edit the article in VE, but it doesn't have the normal look.>>