For what it's worth, you can do that pretty easily with 'xautolock,' and presumably also xscreensaver (which, at heart, is xautolock + an actual locking feature). [xautolock just runs a program, any program, after N units of inactivity.]

But you'd need to write the actual 'hack' (in xscreensaver terminology) that would take over the screen and do all that cool shit.

KDE and Gnome have grown some 'active desktop' clones, so perhaps that's one technique, if it's possible to target those applets at what is not actually the root window... and run an instance of that with xautolock.

Or you could just call your presumably-running instance of mozilla with the necessary command-line mojo to open a new tab (or heck, fullscreen session?) with your nifty-page-o'-stuff on it.

But if you wanted to actually have locking with this (DO NOT WANT PEOPLE LOOKING AT PORN), you'd have to 1. struggle with putting Moz into kiosk mode or something, and 2. use xscreensaver to get the locking functionality, which 3. seems to probably require some arcaneness of its hacks, which could rule out unmodified Mozilla, and 4. if Mozilla causes an OOM condition (which happens annoyingly frequently for me, since I'm the 10 windows/10 tabs per window kind of guy) things could get messy. (xscreensaver has some protections there versus xlockmore, but I think they revolve around trying to *reload* the hack, and trying to reload Mozilla when Mozilla is possibly still busy dying is not my idea of fun.)