The default shortcut for copy is Ctrl+C. If Ctrl+C doesn't work either, perhaps there is something unusual about the selected text. (I created an add-on to treat Ctrl+Shift+C like Ctrl+C, in case it's needed: https://addons.mozilla.org/firefox/addon/ctrl-shift-c-should-copy/ )

See also: *https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly *https://support.mozilla.org/en-US/kb/mouse-shortcuts-perform-common-tasks I notice that you posted with a Snap Firefox version that keeps its profile in a different location and also has different permissions for local file access. You can try Firefox from the official Mozilla server if you currently use a version from the repositories of your Linux distribution to see if it behaves differently. *https://www.mozilla.org/en-US/firefox/all/#product-desktop-release *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile