@eaglejohn: cooperanet's site is also working fine for me in Firefox. I'm pretty sure I'm not being tricked by my own plugin :)
If you have a specific URL that exhibits an issue in Firefox, please post it so it can be properly evaluated. Thanks.
@cooperanet: SLB's lightbox is actually loading in other browsers such as Chrome, but there appears to be a CSS conflict with your site's theme causing the lightbox to remain hidden. One way to narrow down the conflict is to temporarily switch to a default WP theme (e.g. TwentyTwelve) and test the same page you posted in Chrome to see if SLB works then (be sure to clear your browser's cache and refresh).
If SLB works with the default theme without issue then we can investigate where the conflict with your theme is.
If SLB still does not display, then it may be the result of a conflict with another plugin. If you provide a list of the plugins currently activated on the site so that they can be evaluated for conflicts.