In another thread, a user with a similar error message indicated this could be caused by a specific plugin not being activated on the site: the Java Deployment Toolkit plugin (which is always set to "Ask to Activate" in Firefox).

Are your Java plugins up-to-date? You can test on this page:

http://www.java.com/en/download/installed.jsp

Then after doing the Java update (if needed) check the activation status on the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. On the right side, look for:

• Java(TM) Platform SE 8 U101
  • standard permission is "Ask to Activate" but if you want sites to be able to use Java without specific permission you can change it to "Always Activate"

• Java Deployment Toolkit 8.0.1010.13
  • standard permission is "Ask to Activate"; Firefox will not permit "Always Activate" for this plugin

Note: If you do not see Java Deployment Toolkit in alphabetical order, scroll down to the bottom and change its permission from "Never Activate" to "Ask to Activate".

Then try your site again and check for a plugin notification icon toward the left end of the address bar (looks like a small gray Lego block). If you click that, you can give the site temporary or ongoing permission to use any plugins the page has informed Firefox it wants to use.

Does that work?

If Firefox is not detecting the request, try setting the permission using the Permissions panel of the Page Info dialog. You can call that up using any of these:

• right-click a blank area of the page and choose View Page Info > Permissions
• (menu bar) Tools menu > Page Info > Permissions
• click the padlock or "i" icon to the left of the site address, then the ">" icon, then More Information > Permissions

Near the top, you'll find "Activate Plugins" and you can select Allow for the Java plugins. Then reload the page and see whether it kicks in.

Any luck?

If we can't get Firefox to work, can you use Internet Explorer 11 for your calendar or does that also fail?