I made a lot of progress but could spend only 3 hours on it during the week-end.

  • resolved: the toolbarbutton I create when the user subscribes to a webslice opens a popup when clicked ; when such a popup contains an iframe, clicking on a link or form submission button in the iframe's html document does not close the popup
  • resolved: I finally succeeded closing the popup when a link is clicked in the popup's iframe but this leads to a focus problem in the window containing that popup element
  • resolved: the whole thing is so different from normal URLs or standard feeds that I can't really use places for the time being, we'll see later. I need a toolbar with the chevron like the personal bookmarks toolbar

Implementing something like WebSlices requires a bit of technical knowledge but as Asa said, it's really a great example of Ze Power Of Mozilla. In the end, I think my implem is going to beat IE8's.