Popups not blocked
From MozillaZine Knowledge Base
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Mozilla browsers will block most pop-up and pop-under advertisements by default. (These are regarded as the same thing in this article.) If you are getting unwanted popups, try the following suggestions.
Your first step should be to make sure that the internal popup blocker is enabled in your browser and that you haven't "whitelisted" the sites that are giving you popups:
Conventional popups can be blocked using the Popup Blocker Settings extension. (The "Block all" setting will block legitimate and unwanted popups alike. You can allow sites to show popups by entering them in "Allowed Sites".)
Hover ads are a special form of popup that cannot be blocked by using the Popup Blocker Settings extension. You can use either NoScript or Adblock to prevent them.
Popups generated by other software
Sometimes other software that has nothing directly to do with Mozilla browsers can cause windows to pop up. Sources of such non-browser popups include the following:
Spyware on Windows
Pop-ups may be caused by spyware, a collective term for a type of malicious software that gathers information about the user. Spyware mainly affects Windows users and can be installed without your knowledge. Some recommended free spyware detectors and cleaners include:
If these don't find it or can't clear it, post in one of these forums for specialized malware removal help:
Messenger Service on Windows
In their initial state, Windows NT, 2000 and XP SP1 allow anyone on the Internet to initiate pop-up messages on your screen via the built-in Messenger Service (not to be confused with the Windows Messenger or MSN Messenger instant messaging clients). The title of these popups is always "Messenger Service". It is recommended that you disable this service. (This service is disabled by default in Windows XP SP2.)
Other software disables the popup blocker
The McAfee Privacy Service software is reported to disable your Mozilla browser's popup blocker, even if that software is uninstalled and you recheck the popup blocker option in your Mozilla browser. This happens because McAfee's software has set the related preference "dom.disable_open_during_load" to "false" via the user.js file and the popup blocker is again disabled when you restart the browser . You can fix this as follows:
If the software is no longer installed, open the profile folder and delete (or rename) the user.js file. Alternately, if you don't want to delete the file because it contains other customized settings, open the user.js file in a text editor and delete the offending user_pref("dom.disable_open_during_load", false); preference line. When you restart your Mozilla application, you will then be able to recheck the popup blocker option and the setting will be retained. Note 1: this may still not enable your popup blocker if McAfee has also scribbled its text in other files. If Firefox still comes up with the popup blocker disabled, then do a file contents search for the text string dom.disable_open_during_load in all files in the Firefox directory (and subdirectories). Note 2: In some cases, even Note 1 will not fix things. If still stuck, do a similar string search in all files of the C:/Documents and Settings directory (and subdirectories). If you find such a file with this text string, delete it.
If the software is still installed and you try deleting the user.js file or the offending entry, it will be added back when the McAfee Privacy Service next starts (e.g., on reboot or when restarting the service). This applies even if you've told McAfee not to block popups. The solution (reported here) is to allow the 'controlling' program, McAfee, to grant popup-blocking privileges, as follows:
Popups from plugins
Error alert popups
By default, Firefox (1.5 and above) and SeaMonkey use error pages instead of "error alert popups", such as "The operation timed out..." and "The document contains no data". See the article browser.xul.error_pages.enabled for additional information.
More effective pop-up blocking
(Windows users) You should also install a hosts file on your system which will help with all browsers and applications, see Blocking Unwanted Parasites with a Hosts File. This method is primarily directed at the really bad sites, the most frequently encountered annoying ads, and sites known to try to modify your system.