lightbox-2-wordpress-plugin doesn’t work properly on Firefox

Are you using No-Script? If so you need to allow the page to execute Javascript. If for some reason Javascript is just not simply working in Firefox then you may have another plugin causing the problem. Perhaps try disabling them all and restart to see if it works. If so then enable your plugins one at a time until you find out which one is causing the problem. You can do this by utilizing the safe mode option as well without having to manually disable everything one at a time.
Another (and immediate) option is to create a new, clean profile. Go into your Firefox folder (most likely c:\program files\mozilla firefox) and in a DOS window type “firefox -profilemanager” and create a new profile. You can always go back to your current one. If it’s working now reinstall the plugins you want and test the script one at a time. It can be time consuming if you have a lot of plugins but it’s an effective way to solve the problem.
Hope that gives you something to start with.

I had to clear my cache on Firefox. I noticed the issue when I would try to access the .js files related to Lightbox directly. It kept referring me to a WordPress “Page not found” error. I also noticed the HTTP response was 304 (Not changed). I cleared the cache and, lo and behold, Lightbox worked.

Adding the lines

<script type=\"text/javascript\" src=\"".$lightbox_path."builder.js\"></script>
<script type=\"text/javascript\" src=\"".$lightbox_path."effects.js\"></script>

to lightbox2.php did the trick for me. You can use the built in editor in WordPress to edit the lightbox2 plugin, just find the spot where the other .js files are included.