It sounds as though you kept your Firefox data (profile folder) through the uninstall/reinstall. Let's see whether we can clean out the problem. Here's my suggested procedure for tracking down and cleaning up bad add-ons, hijackers, and ad injectors. I know it seems long, but it's not that bad.

(1) Open the Windows Control Panel, Uninstall a Program. After the list loads, click the "Installed on" column heading to group the infections, I mean, additions, 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.

(2) Open Firefox's Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons
• in the Windows "Run" dialog, type or paste
  firefox.exe "about:addons"

In the left column, click Plugins. Set nonessential and unrecognized plugins to "Never Activate".

In the left column, click Extensions. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions. Bear in mind that none of these came with Firefox, they are all optional.

Often 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.

Any improvement?

(3) To check for other issues, you can supplement your regular security software with the scanning/cleaning tools listed in our support article: Troubleshoot Firefox issues caused by malware. These on-demand scanners are free and take considerable time to run. If they finish quickly and especially if they require payment, you may have a serious infection. I suggest the specialized forums listed in the article in that case.

Success?

Thanks a lot for replying! [1] Nothing strange was found [this time] in the Control Panel list - but my earlier cleanout of recent installations did NOT remove the pop-ups. [2] There are very few Add-Ons - most added to remove this problem! [3] Likewise the Plugins, except that there were several with 'good provenance' - e.g. installed by 'reputable' programs / sources - so I left them alone. [4] I downloaded ADWCleaner - though I had an earlier version onboard, so maybe I had tried it before...? Either way, I'll try it immediately after this!

I just ran ADWCleaner and it removed 2 'services' - then I restarted the laptop and tried Firefox again. The popups appeared as soon as I opened a non-Mozilla page!

What else can I do?

Could you test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

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

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement? (More info: Diagnose Firefox issues using Troubleshoot Mode)

I've tried each of the remedies suggested - without success! I've currently got all add-ons disabled [except the one for selecting a choice of new tab URL] and all plugins set to 'ask to activate' and this is my third 'clean' re-installation of Firefox. Still the advert-trojan controls what new tabs open when I click on a link or the up-down strip on the right of the window. It's getting to the point where I can't use Firefox at all!

I'm curious what you mean by this --

this is my third 'clean' re-installation of Firefox.

Did that involve renaming (or deleting) the

C:\Program Files (x86)\Mozilla Firefox\

folder in between installations?

Could you do this three-minute experiment:

Create a new Firefox profile

A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should have completely fresh settings databases and a fresh cache folder.

Exit Firefox and start up in the Profile Manager using Start > search box (or Run):

firefox.exe -P

Don't delete anything here!

Any time you want to switch profiles, exit Firefox and return to this dialog.

Click the Create Profile button, assign a name like TEST2015, and skip the option to relocate the profile folder. After creating the profile, select it and start Firefox in that profile.

Any difference?

When returning to the Profile Manager, you might be tempted to use the Delete Profile button. But... it's a bit too easy to accidentally delete your "real" profile, so I recommend resisting the temptation. If you do want to clean up later, I suggest making a backup of all your profiles first in case something were to go wrong.

Ooops! I did NOT remove or rename the folder "C:\Program Files (x86)\Mozilla Firefox\"! Since reading your most recent helpful suggestion, I un-installed Firefox again and removed this folder, restarted the laptop and re-installed. It seems to have kept my profile and is running properly - at least for now!

Believe me, I'm not just being 'ungracious' - the last time I re-installed, the fault came back progressively - with ever higher proportion of 'clicking on links' leading to advert-sites [instead of the intended target].

I hope it stays working! THANKS!!!!!

Adwcleaner usually finds and removes most stuff like this. If it doesn't remove everything you can try Junkware Removal Tool (JRT). It goes deeper. Just Google JRT - it can be downloaded from several sites. Then, of course, run Malwarebytes plus Ccleaner on the registry.

You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.