dkapp said

The only way to leave the site and go to another site selected from my bookmarks is to close firefox and then open it as new.

Hello dkapp,

If it's your home page you want to go back to, you could either click on the Home icon, or press Ctrl + N - would that work for you  ?

To solve the mystery of the greyed out back button, I would suggest to start Firefox in Safe Mode, but according to your system details your extensions are inactive .....

But what caused the back arrow which used to work great to gray out? Did a setting change?

dkapp said

But what caused the back arrow which used to work great to gray out?

I was hoping that an extension would be the culprit, but as they are inactive .....

One thing you could check :

Type in the address bar about:config and press Enter (promise to be careful, if asked)

Type in the search bar browser.sessionhistory.max_entries

The default value is 50 - another user mentioned that the value had changed to "1"; resetting the value to its default, solved the problem ......

If you had to change the value, then close and restart Firefox.

Reinstall firefox account thanks.

Builtin chrome:// URI page like about:home and about:newtab are special pages with elevated permissions. You can't go back to such a page using the back button. You can close the tab and reopen a new tab with the Firefox Home page (click the '+' icon or click the Home button).

You can set this pref to false to prevent closing Firefox.

• browser.tabs.closeWindowWithLastTab = false

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox