When you say "running Firefox on bare X11 in full-screen mode" I immediately assume you are running a *nix locally on this system.
Under this assumption there is an other option: Install Apache locally and simply configure it to proxy the webserver at 'localhost' to the single page you need.
When the network is out or there is an other kind of error you can simply change the local 404, 500, etc. error pages of this Apache.
These 'error pages' can all have a different retry period. Depending on the type of error.
Very simple and no additional programming/plugins needed. All OOTB software.