Need to get an idea of what's starting when you boot up. Use Autoruns and it will show you what's starting. Then use Process Explorer to see what process is driving the pop-ups with large X in the menu bar. Click and hold the X, then let go when your mouse is over the pop-up. It will then show you in the process window, what process is driving the pop up. FUN STUFF!!!  ;-))

https://learn.microsoft.com/en-us/sysinternals/downloads/autoruns https://learn.microsoft.com/en-us/sysinternals/downloads/process-explorer

Those popup windows are on bottom corner in Firefox? if so then they sound like Web Push notifications from one or more sites you interacted with. https://support.mozilla.org/en-US/kb/push-notifications-firefox

Fake McAfee detection popups can be common.

You can also check for malware on your Windows system. https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware