sisplugins is that this sis ? http://www.edutracsis.com/ eduTrac SIS : Plugins http://www.edutracsis.com/

If so, there is a support forum for eduTrac SIS over here: http://www.edutracsis.com/forums/

I doubt if that is a Firefox support issue.

This is no Firefox issue but a firewall issue. To solve the problem, first make sure you trust the application, because sisplugin.exe is sometimes used for malware, especially when it slows down your computer and you didn't install a program that uses sisplugin. But if it's from edutracsis, as edmeister said, you should be OK.

If you're unsure, open the task manager (Ctrl + Alt + Del), search for sisplugin.exe and make sure it doesn't run from a folder like this: https://www.sophos.com/de-de/threat-center/threat-analyses/viruses-and-spyware/Troj~Banker-GBZ/detailed-analysis.aspx .

If it does, scan your computer with an antivirus software and use Hijackthis http://sourceforge.net/projects/hjt/ , then restart your computer and see whether the problem (and application) is gone.

If you trust the application, you need to allow the program to pass the firewall by opening the port 10013. How to do this depends on your firewall.