Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

Is this with all websites/pages, some, a few?

Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

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

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

Note that some websites set specific dimension to elements on their page to get a fixed layout. Some use @media rules to adapt to various screen dimensions.

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))

• https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages