Hi Dropa, Thank you for your reply. The email provider is Yahoo. I tried to send a screen shot but was unable to make the problem happen just when I wanted it to happen so I could send the screen shot. But, the problem occurs when I make a mistake typing out a reply and try to correct the mistake. When I put the cursor next the printed word in question and try to type the correction, a blue line will appear covering sometimes a word or sometimes a whole line. I have to keep fiddling around until the blue line disappears. Best way I can describe it, Dropa. Can you possibly help? Thank you.

Hi Dropa, thanks for the reply. So, the blue highlighting of the print text when I try and fix a error could be a equipment issue more than a Yahoo or browser issue? I suspect the only way to tell is replace the two possible issues you suggest, mouse or keyboard? I believe I have already tried a different browser to see if that was the issue and I believe that didn't solve the problem. I've been trying to show you a screen shot, but wouldn't you know it, I can't get my computer to show the problem when I'm texting a message. Thank you, Dropa. Appreciate your help. I will show a screen shot just to make sure you understand that problem it the problem appears.

This is not the blue highlight you normally see if you select text ?

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Hi cor-el, not sure what you mean when you state, "This is not the blue highlight you normally see if you select text" I don't conscientiously select text when I reply to a email? It is NOT the blue highlight you see when you try and copy something and insert into a text, as I have done here.

  I can see though being in safe mode does not cure the problem, as I have seen the blue image, or maybe better stated a text line or word being covered in blue when typing a text, because that issue is happening right now as I right out this message reply. Also, working without hardware acceleration doesn't solve the issue. By the way, cor-el what does hardware acceleration do? Do I need to enable it again? 
  Is there a way to tell if my issue is equipment ( mouse, keyboard) related or something in a program from a browser or Yahoo or Microsoft? Thank you, cor-el.

Do you only see this with the Yahoo email textbox? Are you sure it's not some kind of editing feature like spell check?

Does it still happen in a new profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile.

Hi zeroknight, thank you for your reply. I will check out what you suggest and get back to you. Thank you, zeroknight.