As a security precaution, change your Facebook password.

https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox

http://www.howtogeek.com/113439/how-to-change-your-browsers-user-agent-without-installing-any-extensions/

thank you for your answer. I changed my FB password yesterday.

I went through the steps of the link you sent, but it wouldn't let me reset the user agent. (the reset was greyed out).

Any other suggestions, please.

Wait and see if there is another try at your Facebook acct.

I have changed the password again this evening, but I'm still not getting email notifications at all. Only ones that tell me I've changed passwords and changed email addresses (which I tried yesterday). I'm really confused as to what is happening. This all started a week ago when FB changed messages to messenger. The emails started getting later and later then stopped. And if I accessed Facebook, via an email, I would get the message that my FB account had been accessed by an unrecognised browser, Firefox on Linux.

Leodog said

but I'm still not getting email notifications at all

Check your setting on site.

It could be maybe somebody is trying to login to your Facebook account whether on purpose or by accident (similar name?) as the user happen to using Firefox on a Linux distro instead of a version of Windows or Mac OSX.

Leodog said

Facebook are sending me emails saying that my account was logged in from an unrecognised browser, then it says Firefox on Linux. As far as I know I haven't got Linux. Not even heard of it.

Linux is an open-source operating system modelled on UNIX and is pretty much free unless you are using some customized option or support. It is extremely popular in use for servers compared to Windows servers. It is used by many to breathe new life in older desktop PC and laptops where the user may not want to pay for Windows 7 or newer. Many even use it as the only OS or in dual booting with Windows on current hardware.

The only thing Windows is good for me now days is for games as Linux support of games and game clients are still more limited but is growing on Steam.

https://distrowatch.com/

FredMcD, my settings are all ok, it's the first thing I looked at.

James, I've been on Facebook for 10 years and this has only just started happening in the last month and as I said, it only coincides with when I go on FB via an email link. I've been on Windows 7 for two years.

You should never enter a website via a link in an email and enter authentication credential without verifying that you are on the right website (click the Control Center 'i' button or padlock) and never enter via an open HTTP connection. It is easy to spoof a link to make it look like genuine.

cor-el, I have been using the 'View on Facebook' link in the emails for years with no problem until recently. I don't enter any authenification via this link. I don't do it on any other link.