I'm using Firefox in Ubuntu with Java to connect to a Juniper Networks VPN. When I upgraded to Ubuntu 13.10 a couple of days ago, the VPN launcher stopped working. I think Firefox 24 came along with the upgrade (that's the version running now).

I upgraded to the latest Java r45 and it still didn't work. Then I noticed a blinking red thing in the address bar where the security lock icon goes. I clicked that and it gave me an option to enable Java for the VPN connection site permanently.

Seemed easy enough to fix. I only had to click that icon once, and it's been working smoothly since.