Cannot debug inline script

My server has a simple cookie check when hit from outside. It’s tiny project I don’t want to go public for now. If there is no cookie I present a user with info message: “You shall not pass. Contact the source for more info.”.

I’ve signed in and I have access to the webpage, but when I open the Dev Tools -> Debugger, I pick “Main Thread -> localhost:31313 -> /do -> smth” and I still see the old text.

Now, I’ve disabled the cookie check and Firefox still shows the old page in debugger despite server not serving it.

How can I fix it?

Is this a cached file problem?