hello cliffhill, please try this: go to firefox > help > troubleshooting information > profile folder - "show folder", then a new window will open up. in this window look out for a file named user.js (it might be used to overwrite your custom settings). in case it is present , delete or rename this file and restart firefox afterwards.

your useragent (the information the browser sends to websites to identify its version) - apparently mistakenly - shows you're on firefox version 3.5. probably some sort of toolbar or adware has overwritten it. please refer to this article in order to reset the user-agent settings in firefox: How to reset the default user agent on Firefox

Philipp, thanks for the suggestion, which I followed. There is no longer a file called user.js in the folder but http://www.mozilla.org/en-US/firefox/new/ still tells me "Looks like you’re using an older version of Firefox."

Outlook.com also refuses to allow me access because of an "out of date" version of the browser.

Cliff

thanks for reporting back, deleting the user.js file was only the first step - please also reset the user-agent settings like it's described in https://support.mozilla.org/en-US/kb/websites-say-firefox-outdated-or-incompatible#w_firefox-is-showing-the-wrong-user-agent & restart the browser.

Philipp,

Sorry, should have read the ENTIRE message you sent. Problem solved - thank you so much. Have a great day.