Web search; https://www.bing.com/search?q=required+privilege+is+not+held+by+the+client

Did you use the normal install or make changes?

Thank you for that search/links. I have tried a normal install and with administrator privileges, and with the anti-virus deactivated.

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

In safe mode with network support the installation progress bar reached about 90% then stopped with the following error message:

Firefox Installer. Hmm. For some reason we could not install Firefox. Choose OK to start again.

I called for more help.

OK, I appreciate that.

Eddie said

I tried to open Firefox yesterday and got the following error message: Mozilla-Firefox 64bit. A required privilege is not held by the client. I uninstalled Firefox thinking that reinstalling would fix the problem but when I attempted to reinstall Firefox I got the same error message. This is on a Windows 10 64bit machine.

Had the same problem; started in safe mode and it was gone. And I also reinstalled Firefox before.

Update: I restored the hard drive from a backup that I made a few weeks ago and that has solved the problem - Firefox is working normally again. I have no idea what caused the problem.

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems.

Solved problem.