Looks like something invisible is on the screen. One possible cause (for example) is the ad blocker blocking the ad, but not the place holder.

Is this with many websites, some, a few?

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Hi. All / any websites, yea. It's not an ad, not a place holder, but a screen shot bit of the first tab I open in a session, which is then superimposed onto all other tabs. It's not part of the website being viewed, it's Firefox superimposing a strip on top. Yes, it's the same phenomenon in Safe Mode, no change, so it seems to be native to Firefox rather than anything to do with an add-on. Screen shot in Safe Mode attached. Moving from full-screen to 'unmaximised' seems to 'refresh' the strip, like it takes a new screen shot.

I called for more help.

Try this: Open a troubled webpage. Mouse to the problem area and Right-Click on it. Select Inspect Element.

Not take and post a screenshot.

The phenomenon seems to have resolved itself somehow. I can't reproduce it now. I don't think I would have been able to right-click on anything as it wasn't an element, it was display issue, but thanks for the suggestion. I'll come back to this thread it if happens again in future.

Here's hoping it stays gone. Please flag your last post as Solved Problem so others will know.