Will Floatbox support viewing images or other content in Fullscreen-Mode, by triggering the corresponding functions of modern browsers?

See Facebook for example...

No, I don't think so. It's not implemented in all browsers, the existing implementations are inconsistent in both presentation and implementation details, and Webkit has random event handling failures after exiting full-screen mode. One of the design goals of Floatbox is that it works well and consistently across all browsers and full-screen behaviour certainly does not currently support that design goal.

There is of course nothing stopping you from implementing your own full-screen behaviour on your own site. The 'fbContent' element of an open floatbox could be the target element of the full-screen request. You could put a clickable widget to kick it off in a caption, header or footer.

Full-screen support is already present for HTML5 YouTube and Vimeo videos, and Flash.

Thanks, that's exactly the reason why I've asked. It wouldn't make any sense to implement this individually if floatbox would add such a feature in a few weeks