Honestly, it would be interesting if there was a little button to click on.
Because, after visiting several websites, I really can't see a little button anywhere in my browser to click. ☹️

I’ve just installed Flagfox https://addons.mozilla.org/nl/firefox/addon/flagfox/
And yes, as already mentioned by user arnofly, this works flawless,
the Flag is visible in the address bar when you open a webpage immediately after installing the extension.