Check in Preferences > Content > Languages: Choose ... if you have "English US" installed. Perhaps you have installed a British English FF version, but you can change that there.

I did what you suggested. It comes up English-UK listed first, thren below English US. I have used the delete feature to remove the UK listing, and I have also used the Move Up feature to put US at the top of the list. I have rebooted many times. I have also re-installed Firefox. It still comes up with the English UK listed at the top each time I go back in.

Sorry, should have read that your user-agent locale shows en-GB.

There may be other ways to change this, but the most direct is to edit the locale settings by typing about:config in the Location Bar (address bar), then enter en-gb or userage in the filter, hit enter, double-click on general.useragent.locale (which currently should have en-GB) to change its value. No reboot necessary, possibly closing and restarting Firefox.

However, there may still be sites that detect your language wrongly (geo-location), e. g. if you don't live in the US, but access a US site (although this is bad site design practice, IMHO).

Alternatively, you may use an extension (https://addons.mozilla.org/en-us/firefox/search/?q=locale+switcher), probably only if you often access sites with problematic language detection.

I do not know what you mean by "user-agent locale shows en-GB". I am in the USA, and want everything set to USA settings.

You currently have an en-GB British Firefox version installed according to your More system details list.
If you are in the US and don't need that British version then download and install the en-US version.

• http://www.mozilla.com/firefox/all.html

Uninstall the current Firefox version, but make sure that you do not remove your personal data. Install the new Firefox version of the wanted language (en-US).