same problem here

No problem in safe mode means the problem could be an add-on.
Start Firefox in safe mode and once it is loaded;
Firefox > Add-ons > Extensions. Go to each add-on and Disable it.

Once you are done, restart FF and see if the problem is still there. If it is gone, go back to the add-ons and re-enable ONLY a few. Restart again. Keep going until you run into the problem again.

I think I solved my problem in this matter with regard to REDIR... script.

I noticed when I was on one of my pin tabs that is when the menacing script message began to pop up. Wherever I went after and even before on the browser, the script would keep popping up. The program was very, very slugish. Everything came to an almost halt. Well one of those script pop ups, besides the REDIR, said the pin tab I was on was Merriment-Webster dictionary. I closed the pin tab and Merriment-Webster altogether, restarted FireFox and its been running like a champ for the last 1/2 hour. I think it is permanently fixed now. That is until the next web page.

Good work. It would be a good idea to contact the programer via the add-on page, and let them know there is a problem.