https://support.mozilla.com/en-US/questions/844091

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

You can find the connection settings in Tools > Options > Advanced : Network : Connection

If you do not need to use a proxy to connect to internet then select "No Proxy"

See "Firefox connection settings":

• Firefox can't load websites but other browsers can

I went to Tools> Options> Content> Choose your preferred language, and made sure I selected English USA. You have to make sure you add the language and move it up to the top of your list. I completely deleted the UK (which I never added in the first place). Now Firefox thinks I'm back in the US and not in the UK.