The back arrow is grayed out when you start Firefox because that is the first page you viewed. There is nothing to go back to. However, once you navigate to a new page it should work. I work with multiple windows (not tabs) all day and the back button always works. Is it possible that you are trying to go back many pages at once and you have hit what I believe is an upper limit to the number of pages you can go back to.

Each new window and each new tab has its own history, you can't go back to a web page that was opened in a different window or tab.

I understand that I can't go back to a site in another tab or window. The problem is that I can't go back to a site that I visited in the same tab and same window. The back arrow is grayed out.

Do you see anything if you right click such a disabled Back button?

Very good! When I right click the grayed out back arrow, I see all the previous sites and can move to any of then with a left click. Don't know why this happens but this is a fix. I did find another fix on my own. I installed a profile manager and used some old profiles. They don't have this problem. I'm still baffled as to the reason for this but your fix works as does working with an old profile.

Thank you!!

You may have an extension that makes the buttons show in disabled state.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems