Go to the Mozilla Add-ons Web Page (There’s a lot of good stuff here), and look for a good add / pop-up blocker.

Thanks, but the issue is deeper than that. It looks like something has hijacked some capability in Firefox and is using it to open several windows in certain web sites. These are obvious malware windows ("Your PC is about to crash!!"). Windows does give me a warning that it has blocked dangerous web sites and gives the IP address. It is only on certain web sites, which leads me to believe it is based on some web page characteristic (e.g. uses frames or some such), My original post was wrong - it does happen with Chrome too. but not IE. That means for now I have to stick with IE. Foo. Ad and pop-up blockers do not fix it.

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!

No joy.

   I ran MS Safety Scanner, MalwareBytes, TDSSKiller and Lavasoft,

but none of them found anything.

   The problem seems to be related to a web site called default-search.net.

Whenever I create a new tab, it opens on this page and several bad pop-ups appear (Windows flags their IP address links as dangerous). I have found a number of web sites which reference default-search.net and give instructions on removing it. However they don't work for me. The page is not listed in the Firefox extensions. And when I change references to it in about:config, that doesn't seem to help. In particular, when I reset or change

     browser.startup.homepage_override.mstone

the next time I open about:config, it has been reset back to 27.0.1. This looks like some process is monitoring the setting and always setting it back, but I couldn't find any suspicious process in taskmanager.

   Another clue - the legitimate pages which show bad pop-ups also

have a vertical window on the left side of the page with the heading Related Searches. The links on this list seem to be related to the main page subject (i.e. medical links on webMD.com). However this window does not appear on IE nor does it appear on Firefox on my laptop. It looks like somebody (default-search.net?) is adding stuff to the web page.

   So it looks like default-search.net has gotten pretty deeply

embedded in my system. Any ideas of what else to look for or how to get rid of it?

Sounds like your Home / New Tab settings were changed.

==========================

Open a new window or tab. In the address bar, type about:config. If a warning screen comes up, press the Be Careful button. This is where Firefox finds information it needs to run. At the top of the screen is a search bar. Enter browser.newtab.url and press enter. browser.newtab.url tells Firefox what to show when a new tab is opened. If you want, right click and select Modify. You can change the setting to;

about:home (Firefox default home page),
about:newtab (shows the sites most visited),
about:blank (a blank page),
or you can enter any web page you want.

The same instructions are used for the new window setting, listed as browser.startup.homepage.

http://botcrawl.com/remove-default-search-net-virus/

MalwareBytes should have found that garbage.
Why try MalwareBytes support.