So it is real simple to get to the options. Did you click the icon and look in there?

I do not understand: So it is real simple to get to the options.

Yes, I can click the icon and can explore all the options in this window. But I find no option to stop keep the window from re-appearing.

Now, I could upload the screenshot.

Try this, remove Ghostery. Then re-boot. Now start Firefox. After everything is done booting, reinstall Ghostery latest version.

I did as instructed (actually a quite logical procedure), but it did not change the situation: as soon as Ghostery is reinstalled, we are at back at square one...

It could be the work of one of your add-ons. Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control>(Mac:<Command>)<Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable all of your add-ons except Ghostery

Restart Firefox. What happens?

...same reaction, even if Ghostery is the only enabled extension.

(Only if all extensions including Ghostery are disabled, there is no pop-up)

To clarify: I am starting with a blank page when Firefox starts. Then I enter an Internet address. e.g. www.linkedin,com. The moment I hit <CR>, the pop-up appears. I normally hit the hyperlink "Dismiss" and the pop-up disappears. If I then finish reading the page and delete the browser address line, and enter another address, the pop-up never reappears again --> only after sending the first address line to the browser after startup.