Sorry, I don't think so. I can close tabs in a FF window but there are additional windows, I can't close them.
Look, I'm going to the site Link Removed to watch football live streams.
Then in that site there's some links to ads with two "x" on it in order to close them.
One of these "x" is a fake one and this opens one or more new windows.
In former times it just opened new tabs in the same window and I could close them.
I'm sending you two screen shots.
The "X" in the right upper corner closes the BitBox window.
I can imagine these locked windows are designed like this (not being editable) by the html programmers.
But the missing "X" for the firefox window might be a BitBox design,
which proves a drawback in such situations.
But there might be a possibility to close FF windows too.