Hello   David,

Could this be what you are talking about (?):

https://malwaretips.com/blogs/remove-coupons-popup/

If so, go to the Firefox menu (≡) => Add-ons => Extensions and remove anything that looks unfamiliar or suspicious.

Next step : run malwarescans;   further information can be found in this article :

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners They all work differently;   what one program doesn't pick up, the other might.

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

You can check for recently installed suspicious or unknown extensions.

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

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Remove the Cookies" for websites that cause problems:

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Clear Data -> Cached Web Content: Clear

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

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

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems
• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop