Does this appear when you visit particular websites (including, possibly, your home pages), or does it seem random or periodic (e.g., every 30 minutes)?

If it is generated by a website, it may be designed to evade the pop-up blocker and you might need to take other steps to block it. For example, there are ad filters, script filters, and other add-ons that might help.

If it is generated by an add-on, then you might need to disable or remove the add-on. You can review those here:

orange Firefox button (or Tools menu) > Add-ons > Extensions category
orange Firefox button (or Tools menu) > Add-ons > Plugins category

Occasionally pop-ups are generated by external software: they can launch a link in your default browser. If you notice this pop-up when Firefox is idle, I would be very concerned about possible malware.

Does any of that tend to point toward a particular culprit?

This is where one of the pop ups is coming from http://ad.adserverplus.com

I have everything disabled and now it is on IE too?

You may want to run some scans for adware on your computer. These three tools are highly regarded (and free):

• Malwarebytes Anti-malware : http://www.malwarebytes.org/products/malwarebytes_free
• SUPERAntiSpyware : http://www.superantispyware.com/
• AdwCleaner : http://www.bleepingcomputer.com/download/adwcleaner/ (ignore banner ads for other products)

this problem recently begun for me as well...seemingly with the last crash/update from firefox browser. the notice changes but each time it's a pop-up and it states that the plugin is not detected or similar.

this looks like a problem caused by adware/malware on your pc. please go to firefox > addons > extensions & remove any suspicious entries (toolbars, things that you have not installed intentionally, don't know what purpose they serve, etc). also go to the windows control panel / programs and remove all toolbars or potentially unwanted software from there and run a full scan of your system with the security software that you have in place and different other tools like the free version of malwarebytes & adwcleaner.

Remove a toolbar that has taken over your Firefox search or home page Troubleshoot Firefox issues caused by malware

Thanks Philipp.. that did it...there was an extension called 'ccontinuetosave'. I removed... not sure how it got there... I ran adware full scan on my pc yesterday and was in the process of running malwarebytes when your response came in... soooo thanks!!