If this doesn't happen on every website, they are probably popups. Regardless of whether or not you have an ad blocker installed on FIrefox, you may still get popups on certain websites.

Are they any specific websites? For example, are they websites that you've visited in the past or is there some specific reason that those websites would be opening?

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