I just sent this support request through Netvibes’ feedback page. Looks like I forgot to mention that I’ve tried clearing Firefox’s cache as well.
Update: I’ve tracked down the cause: disabling AdBlock Plus from Firefox’s extensions restores the Add content panel’s contents. There’s nothing related to netvibes.com in my filters list so I have yet to find out which exactly is it that’s blocking the panel when AdBlock is in use. I’ve heard nothing from Netvibes support, so I’m not bothering to report this to them either.
After migrating to Ginger, since the beginning of this week I’ve been unable to add content directly to my Netvibes Universe page at http://netvibes.com/jani
The problem is that the upper portion of the screen, where adding new content normally takes place, is blank when I bring it up with the ‘Add content’ (which is ‘Lisää sisältöä’ in my localized view) button at the top of the screen. To clarify what I mean by this I’ve created a couple of snapshots of the Universe page with the Add content panel showing (or rather, not showing):
At first  even the closing button [×] is missing. When I select ‘Add content’ from the top of the screen a second time, the closing button now appears , but the actual content adding widgets are still missing. A third cli[c]k on ‘Add content’ closes the (still empty) adding panel.
Contrast this with the content adding widgets being present, as they should, on my private page, when I click ‘Add content’ there: http://mummila.net/nuudelisoppa/files/2008/03/Inttervepin-Ihimemaa_1205681961397.png
Due to this issue I’ve had to resort to a bit of juggling: when I want to add something to my Universe page, I go to my private page and ‘Add content’ there. When the new widget (for example, a subsciption to a feed) appears on my private page, I then use the ‘Send’ item on its top frame to send the widget over to my Universe page. Finally I delete the widget from my private page.
I’m using Firefox 18.104.22.168 in Ubuntu (Gutsy Gibbon). Before submitting this report, I clicked on the ‘clear cache’ link on the feedback form page, tried adding content again and found that the issue remains.
I’ll be happy to provide you with any additional data you might need in order to solve this. Apart from this issue I’ve been very satisfied with Netvibes, and am particularly happy with Ginger.