There's a good chance these are injected by an unwanted add-on. Try this:

Disable 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.

Does that flush the ads?

By the way, if you want an opinion on whether your extensions look legit, you can grab a copy from the Troubleshooting Information page, either:

• "3-bar" menu button > "?" button > Troubleshooting Information
• Help menu > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the upper left corner you will find a button to "Copy text to clipboard" that you then can paste into a reply. There also is a lot of other information starting from "Important Modified Preferences" that might not be relevant to your current issue so you could remove that from the post if you like.

I only had 4 extensions, all ad blockers. I got rid of them, but the skipit video popups persist.

Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Did this fix your problems? Please report back to us!

You also can check on whether your Firefox connection setting was hijacked to route your requests through a different internet service.

"3-bar" menu button (or Tools menu) > Options > Advanced > Network mini-tab > "Settings" button

The default setting is "Use system proxy settings", which piggybacks on your IE settings, but you also can try "No proxy" to see whether that helps.