This is a screenshot of what I see.

Hi   !

Do you by any chance have an add-on installed, called 'Tab Groups 2.14'  ?

If so, would you disable it and see if that makes a difference  ?

If that's not it :

Let's try Firefox in Safe Mode : If Firefox is not running : Hold down the Shift key when starting Firefox. If Firefox is running : 3-bar menu => ' ? ' (at the bottom of the dropdown) => 'Restart with Add-ons disabled'. Then OK the restart. A small dialog should appear. Click 'Start in Safe Mode' ( not 'Refresh' !) See : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

And also : https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thanks for this, but as I mentioned, the letters are hugely enlarged if you look at the screenshot. I therefore do no have any access to the drop down bar.

richard777 said

Thanks for this, but as I mentioned, the letters are hugely enlarged if you look at the screenshot. I therefore do no have any access to the drop down bar.

You don't need to have access to the drop down bar - would you read my post again : there it's explained how to start Firefox in Safe Mode.

I take it that you don't have the Tab Groups add-on  ?

When it turns out that on of your add-ons is the culprit and you need
to re-enable them one by one : Can you type in the adress bar : 'about:addons' to get there  ?

You may have accidentally zoomed web page(s). Reset the page zoom on pages that cause problems.

• View -> Zoom -> Reset (Ctrl+0/Command+0 (zero))

• http://kb.mozillazine.org/Zoom_text_of_web_pages

Try to start Firefox in Safe Mode to see whether that works.

• Windows: hold down the Shift key while starting Firefox
• Mac: hold down the Options key while starting Firefox

Make sure you do not run Firefox in Full Screen mode (press F11 or Fn + F11 to toggle; Mac: Command+Shift+F).

• https://support.mozilla.org/en-US/kb/how-to-use-full-screen