Calendar gridlines missing in Firefox 26.0

This appears to have something to do with something Firefox doesn’t like about your theme’s CSS for table formatting. Seems to be related to the border-collapse property.

You can add some custom CSS like this:

table#cpcalendar
{
border-spacing: 1px;
border-collapse: separate;
}

or play around with styling on the table#cpcalendar td, table#cpcalendar th properties for table cells and heading cells. You can either add this to the CSS for your theme or register a different stylesheet from the default included with the plugin on the settings screen.

You can see the default stylesheet for the plugin here https://rsvpmaker.com/wp-content/plugins/rsvpmaker/style.css

I don’t specify formatting for the calendar table, so you get whatever the default is for your theme unless you specify something different.

The CSS code snippet you suggested worked perfectly. After adding it to the theme CSS the table displayed perfectly. Thanks very much for the quick and accurate response.