I doubt the No script addon got infected. However, try the following:

First, try to reset Firefox. Refresh Firefox - reset add-ons and settings. This simple step will eliminate many Firefox problems with only one click.

Download and Install MalwareBytes Anti-Malware, run a full Scan. http://www.malwarebytes.org/. This helps check that there are no viruses on your computer causing issues. You can uninstall this program after you clean off any infections.

Hi Tyler, Thanks for quick reply

Initially after some troubleshooting I noticed that I could make infection pop up disappear:

From Configure how firefox connects to the internet -> changed from "use system proxy settings" to "auto-detect proxy settings" or "no proxy"

With this option modified, the pop up from avast didn't appear any longer when opening Firefox.

However I noticed that both Chrome and IE were set to work on the same "use system proxy settings" and they weren't causing problems...so I assumed the problem still has to be with firefox.

After changing back the option in firefox to "use system proxy" I got again the notice, but saw that when opening firefox in safe mode the issue was gone again.

I tracked it to the no script addon, after removing it, the issue disappear regardless of proxy config settings...when I reinstall it, it appears again.

I am now running in depth virus scans to see were the actual infection is.

It seems to me it is expoiting a vulnerability in this addon, I will try the reset also.

Managed to fix issue, after in depth scan with avast free edition detected and deleted 23 infections.

I am all clean now, installed noscript back and no more issues.