In case one of your extensions is involved, could you test in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• (menu bar) Help > Restart with Add-ons Disabled

Not all add-ons are disabled: Flash and other plugins still run

After Firefox shuts down, a small dialog should appear. Click "Start in Safe Mode" (not Reset).

Any difference?

Also, hopefully you installed from Mozilla's site and not one of the bundles advertised in Google search results that sneak unwanted software onto your system.

Just in case, could you check the Windows Control Panel, Uninstall a Program. Click the "Installed on" column heading to group the items by date. This can help in smoking out undisclosed bundle items that snuck in with some software you agreed to install. Take out as much trash as possible here.

You also might supplement your regular security software with some of the scanning/cleaning tools in our support article: Troubleshoot Firefox issues caused by malware.

There is no difference in "safe mode", and I did download from Mozilla's site. Not sure if "ads" is the right term. Some say "stay connected on Sprint", "Your PC is about to crash. click OK to fix, "PC support chat - Microsoft specialist 'is your PC experiencing problems?'" Seems more like spam or pop -ups. Tried un-installing Firefox, then re-loading , but it's the same.

Could you check:

(1) Potential malware - previous post: https://support.mozilla.org/questions/1014988#answer-614380

(2) Modified connection settings? If you open this dialog:

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

the default setting is "Use system proxy settings" which should piggyback on your IE/Windows LAN setting. You also could try "No proxy" to see whether that helps.

(3) Is it possible that you are browsing in a framed page instead of using the full tab? If you right-click your actual content page, does "This Frame" appear on the right-click context menu? If so, click that to expand the little menu and Open Frame in New Tab. Does that one have the crappy content around it?

If you just happen to visit sites with a lot of advertising, and you find it annoying, you could consider using an ad blocking extension. This one seems most popular: https://addons.mozilla.org/firefox/addon/adblock-plus/ (I don't use it myself)

Tried it all, but it doesn't help. New info, however: just realized I have Firefox 31.0 on my old computer, and it works just fine. Must not be Firefox. The old computer is an HP running Windows 7. The new one is also an HP running Windows 8.1.

Can you compare the same pages in Internet Explorer on your Windows 8.1 system? Does Firefox have extra ads/pop-ups or the same ones?

Thanks for that tip. I get the same adds on the same pages in IE. Now I need to find out how come.

Did you scan for malware on your computer using malwarebytes and the other tools we suggested? this is a common sign of a malware infection

Thank you. When I said I had tried "all of that", I had overlooked "malwarebytes". It seems to have cleared the problem.