There is a way to import them but it's not perfect. It dumps everything into one group. But it least its a start
I opened up the opera bookmark browser, and highlighted the bookmarks i wanted, I then right clicked the highlighted bookmarks, and selected "file", then "save selected as html". If you just export as html, you'll get all these opera default bookmarks in addition to your own bookmarks. Opera creates an opera6.html. Which for some reason doesn't import into internet explorer or firefox. I had to edit the opera6.html file
I got rid of the header information.
At the top is something that looks like this.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title>
Bookmarks
</title><style type="text/css">
dl { margin-left: 2em; margin-top: 1em}
dt { margin-left: 0; font-weight: bold}
dd { margin-left: 0; }
dl.first {margin-left: 0 }
</style>
</head>
I deleted this and then saved the file.
and I was then able to import it into firefox. It just came in as one big list, no folder though.
Hope that helps