browser.tabs.remote.autostart = false
   browser.tabs.remote.autostart.2 = false 

   http://www.ghacks.net/2016/07/22/multi-process-firefox/ 

   http://kb.mozillazine.org/about:config 

I actually asked this same question in January of this year. This is the text from my original question: Quoted text start: I have installed Multi-fox on my windows 8.1 PC about a year ago. It was working fine until today, it suddenly showed an error that reads, "Multifox doesn't work when multiprocess mode is enabled..." What does this mean? Can I disable multiprocess mode? If not, what can I do to fix this problem. The error message said something about it won't be updated because "The development time would not be worth it..." I hope that multifox is gone forever. Any information would be appreciated. End Quoted Text The following is the reply that ultimately worked and solved my problem Quoted Text start: You can disable multi-process tabs in Firefox by setting the related prefs to false on the about:config page. browser.tabs.remote.autostart = false browser.tabs.remote.autostart.2 = false http://www.ghacks.net/2016/07/22/multi-process-firefox/ You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue. http://kb.mozillazine.org/about:config End Quoted text This problem has occurred once again. When I went into the about:config page to repeat the process I discovered that these options were already set to false, but I'm still having the problem. Now I'm lost.