Why am I suddenly getting pop up adds when I make selections on the browser?

This can happen if an extension or unwanted software is injecting ads into Firefox. Try these steps: 1. Go to Menu → Add-ons and themes → Extensions and temporarily disable your extensions to see if the pop-ups stop. 2. Check Settings → Privacy & Security and make sure pop-up windows are blocked. 3. Try Firefox Troubleshoot Mode to see if an extension is causing the issue. 4. If it continues, check your computer for unwanted software. Do the ads appear on every website or only on certain sites?

https://support.mozilla.org/en-US/kb/push-notifications-firefox#w_how-do-i-revoke-the-notifications-permission-for-a-specific-site

Are these as small popups on right side of Firefox window?. If so then these may just be web push notifications from sites you have allowed to send you notifications. Instead of useful information some may send you junk, spam, fake virus/trojan/antivirus alerts etc. You can revoke/block these sites (mentioned in popups) from sending you notifications.