"Change the Default Search Engine" shortcut keys do not work on Firefox 140
According to https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly?redirectslug=Keyboard+shortcuts&redirectlocale=en-US#w_search , cha… (read more)
According to https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly?redirectslug=Keyboard+shortcuts&redirectlocale=en-US#w_search , changing the default search engine should be Ctrl + ↓. The shortcut key has changed in Firefox 140, but the description page has not been updated.