Suggest try turning/disable of all your Extensions and try Firefox with all off. Make sure all Extensions are up to date.

Slowly start turning each one back on will browsing then turn on another.

This will determine if one of the Extensions is to blame or not. https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems?redirectlocale=en-US&redirectslug=Troubleshooting+extensions+and+themes

Then please follow this https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

Could also try this https://addons.mozilla.org/en-US/firefox/addon/fat-free-firefox/

Please let us know if this solved your issue or if need further assistance.

When multi-process windows in Firefox is enabled then Firefox will use more memory and may be less responsive. Try to disable multi-process windows in Firefox to see if that has effect.

You can disable multi-process windows in Firefox by setting these prefs to false on the about:config page.

• browser.tabs.remote.autostart = false
• browser.tabs.remote.autostart.2 = false

You can open the about:config page: via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

• http://www.ghacks.net/2016/07/22/multi-process-firefox/

Checking the extensions I found that in one list Firefox showed the presence of 'Pocket' while it was absent from the "about:addons" list (I had installed Pocket in a previous release, didn't like it, uninstalled). So, unable to remove Pocket again, I examined the extensions with IObit's Uninstaller, which found and removed the remains of Pocket.

At the moment, I don't know if that has made a positive difference.

Concerning "about:config", browser.tabs.remote.autostart was already set to false and I have set browser.tabs.remote.autostart.2 = false

Again, I do not know yet if these changes have made a difference, time will tell.

Thanks very much Pkshadow and cor-el for your help, much appreciated.

Note that Pocket is one of the built-in extensions that are installed automatically and that only show on the Help -> Troubleshooting Information page and not on the about:addons page.

You can disable Pocket by moving the Pocket toolbar button to the Customize palette. That should remove all Pocket menu entries.