How do I block an ad that opens up a new Firefox window and keeps telling me my system performance is poor? I can't even capture the link.

Question

This particular ad, supposedly from a Windows Certified Partner, opens in a second Firefox window and keeps flashing the message that my System Performance is Poor, wanting me to click on their ad and let them do whatever nasty things they want to my system. I have not been able to capture the path but next time it comes up, I will write down all that I can see.
If I have closed my main Firefox screen first, and then close out this ad, the next time I open Firefox, the ad opens and all my standard tabs are gone, requiring me to reopen each one seperately (Facebook, Google, etc).

Answer 1

Just to address the last point: check your History menu to see whether you have the Restore Previous Session option and use that if you can. If that is grayed out or doesn't restore everything, check the Recently Closed Windows and Recently Closed Tabs lists for other pages.
The unwanted window may be generated by an add-on. Try disabling ALL nonessential or unrecognized extensions on the Add-ons page. Either:
* Ctrl+Shift+a
* "3-bar" menu button (or Tools menu) > Add-ons
In the left column, click Extensions. Then, if in doubt, disable.
Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.
Any improvement?
Here are some other things to check:
(1) user.js file that changes Firefox startup behavior and overrides your preferences. This article describes how to track that down and delete it if you have one: [[How to fix preferences that won't save]].
(2) Possible hijacked shortcut. Check the "target" of the desktop icon you use to start Firefox to see whether it lists the unwanted page. To do that:
right-click the icon > Properties > Shortcut tab
For 64-bit Windows 7, the Target should be no more and no less than this:
"C:\Program Files (x86)\Mozilla Firefox\firefox.exe"
(3) Possible undisclosed bundle items. If you have installed any free software recently, check your Windows Control Panel, Uninstall a program for surprises. If you click the "Installed on" column head to group by date, it is easier to spot bundled junk. Remove everything suspicious or unrecognized.
(4) Supplemental clean up scans. Our support article lists tools other Firefox users have found helpful: [[Troubleshoot Firefox issues caused by malware]].
Hopefully that cures it.