The image looks okay to me. Is this with all websites/pages, 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?

You may have switched on caret browsing. You can toggle caret browsing off/on by pressing F7 (Mac: fn + F7).

• Options/Preferences -> General: Browsing
  remove checkmark: [ ] "Always use the cursor keys to navigate within pages"

Note that this is a Firefox accessibility feature.

• https://support.mozilla.org/en-US/kb/accessibility-features-firefox-make-firefox-and-we