Hello, '''Try Firefox Safe Mode''' to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons. ''(If you're not using it, switch to the Default theme.)'' * On Windows you can open Firefox 4.0+ in Safe Mode by holding the '''Shift''' key when you open the Firefox desktop or Start menu shortcut. * On Mac you can open Firefox 4.0+ in Safe Mode by holding the '''option''' key while starting Firefox. * On Linux you can open Firefox 4.0+ in Safe Mode by quitting Firefox and then going to your Terminal and running: firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox) * Or open the Help menu and click on the '''Restart with Add-ons Disabled...''' menu item while Firefox is running. [[Image:FirefoxSafeMode|width=520]] ''Once you get the pop-up, just select "'Start in Safe Mode"'' [[Image:Safe Mode Fx 15 - Win]] '''''If the issue is not present in Firefox Safe Mode''''', your problem is probably caused by an extension, and you need to figure out which one. Please follow the [[Troubleshooting extensions and themes]] article for that. ''To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.'' ''When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.'' Thank you.

Further to the first reply, when I search your Plus-HD extension in Google, the results seem to confirm that it causes popup ads. Do you recall installing this?