Ctrl+c with text selected in a page, or somewhere in the user interface (for example, in the address bar)?

If it's in a page, it's possible that the page has overridden the normal action for Ctrl+C.

However, if it happens on numerous different sites, an add-on that injects an interceptor script into web pages or even a hotkey program outside of Firefox could be involved. To distinguish between those, try copying text in a PDF you are viewing in a tab. Extensions are not allowed to inject scripts into the PDF viewer.

Thanks for your reply, jscher2000!

Based on your information, it must be an interceptor script or a hotkey program, since it happens when I select test in the web page, not the address bar. I doesn't happen when select text in a pdf displayed in a browser window, or in the Firefox settings window.

I'm not sure what to disable. Britannica.com probably isn't the source of the problem and I'm not sure what program/script/etc would direct copied text to Britannica's search.

I'll scrutinize my extensions...

Found it!

It was Context Search https://addons.mozilla.org/en-US/firefox/addon/contextual-search/

I uninstalled it. Even though the Britannica search wasn't checked to be active, turning the extension off and on made the problem go away and come back.

Thank you for reporting back. The author says there should be a way to delete the assignment of Ctrl+C to Encyclopedia Brittanica:

https://github.com/odebroqueville/contextSearch/issues/153