/Applications/Firefox.app/Contents/MacOS/firefox -new-tab https://www.mozilla.org

If I run /Applications/Firefox.app/Contents/MacOS/firefox -new-tab https://www.mozilla.org when Firefox is already running, I get the error "A copy of Firefox is already open. Only one copy of Firefox can be open at a time." It is true that Firefox is already running, but what I am shooting for is getting a new tab in the already running instance, as the -new-tab flag indicates. I get this problem even if the already running instance was started with exactly the same command. Some of the flags at https://developer.mozilla.org/en-US/docs/Mozilla/Command_Line_Options explicitly say they open new tabs or windows within an existing instance of Firefox, such as -private-window, but I get the same behavior with those flags. What am I doing wrong?