Since the computer is not yours, why not use a Private Window?

You’re in a Private Window

Clears your search and browsing history when you quit the app or close all Private Browsing tabs and windows. While this doesn’t make you anonymous to websites or your internet service provider, it makes it easier to keep what you do online private from anyone else who uses this computer.

I'd not thought of that. Thanks for the insight.

Though it is a problem with FF saving passwords and auto-logging into websites when it's specificly selected not to remember in the pop-up window.

Thanks for your help.

Thanking for shopping Mozilla Support, have a good day.  ;-))

Note that if you uninstall Firefox then you likely keep the Firefox profile folder with your personal data.

If you logout on those websites like you always should then others shouldn't be able to login. Some websites have a checkbox to log you in automatically and this choice saves the session ID in a cookie. Private Browsing mode like suggested above will prevent keeping these special cookies.

Note that you should never connect to Sync on a computer that isn't your own because data loaded from the Sync server will still be saved on the computer when you disconnect Sync.

You can also consider to use the portable Firefox version. The portable version comes with its own profile folder and doesn't interfere with currently installed Firefox versions.

• https://portableapps.com/apps/internet/firefox_portable

Thanks cor-el.

I just tried signing out of all the re-opened tabs on starting up FF, closed FF and then re-started it. The least secure was this FF site, which allowed me to sign in with a pre-saved account email address and without asking for the password.

The other sites required my passwords upon re-starting FF.

This has just started occurring since I had a new mobile phone which is capable of running apps for my accounts.