hello, this sounds like a problem possibly caused by adware on your pc.

1. please go to the firefox menu ≡ > addons > extensions & remove any suspicious entries (toolbars, things that you have not installed intentionally, don't know what purpose they serve, etc).
2. also go to the windows control panel / programs and remove all toolbars or potentially unwanted software from there.
3. finally, run a full scan of your system with different security tools like the free version of malwarebytes & adwcleaner, which are specialised in adware and browser hijackers.

Remove a toolbar that has taken over your Firefox search or home page Troubleshoot Firefox issues caused by malware

If the above doesn't help, could you copy/paste the full address of the unwanted page? Sometimes the parameters on the address point to the culprit.

Also, does this occur if you launch a new window using Ctrl+n?

If yes:

Every new window uses your home page setting. It's a little too easy to end up with multiple "home tabs". To check on that, you can use the Options page as described in this article: Startup, home page, tabs, and download settings.

Look for a vertical bar character in the middle of the field separating two addresses. As a simple example with short URLs:

https://www.mozilla.org/|https://search.yahoo.com/

The bar definitely can be hard to spot with long URLs, but when you find it, delete everything to the right of the bar and then the bar itself (I suggest deleting the bar last so things don't accidentally get mushed together).

Does that help?

phillipp I tried the first two suggestions, with no luck. Nothing untoward shows in either my addons or my control panel programs. I am downloading malewarebytes right now, so I can try it.

jscher2000

I tried changing the entire address in the options as you've described, but the page keeps coming back. This is the full address of the page......

https://search.yahoo.com/?type=994519&fr=spigot-yhp-ff

It does happen every time I ctrl-n as well as when I start firefox after having closed it completely. Creating a new tab or new window with Firefox already open, does not trigger it.

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control>(Mac:<Command>)<Shift> A)

In the Add-ons Manager, on the left, select Extensions. Remove Yahoo Anything. Now do the same in the computers Programs Folder.

Windows: Start > Control Panel > Uninstall Programs. Mac: Open the "Applications" folder. Linux: Check your user manual.

After the problem is fixed, Bookmark and use this; Download the Mozilla Search Reset {web link} This add-on is very simple: on installation, it backs up and then resets your search preferences and home page to their default values, and then uninstalls itself. This affects the search bar, URL bar searches, and the home page.

You can check the target line in the Firefox desktop shortcut (right-click: Properties) to make sure that nothing is appended after the path to the Firefox program.

Do a malware check with several malware scanning programs on the Windows computer.

• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked

fredMcD There is no mention of yahoo anything in either of those places.

Phillipp I have run Malwarebytes, and it did locate issues, which I removed and restarted, but the problem persists.

cor-el Nothing extra appears to be appended when checking the shortcut properties. It looks like I will be installing all of these and trying your method next.

Reikokage said

This is the full address of the page......

https://search.yahoo.com/?type=994519&fr=spigot-yhp-ff

Spigot is a classic source of unhelpful software. Hopefully one of the cleaners will find the mystery item and take it out.

Ran all the programs cor-el suggested, but they found nothing. The problem was still occurring.... I did all the browser clean-up I could: deleting cookies/history, reset to default homepage...

Thinking about how cor-el said something could have been appended to the shortcut... i also removed the firefox pin and app. (since I'm on windows 10) I then replaced them all with new shortcuts via the firefox in the programs folder. This finally solved the issue!

So for anyone else with this problem, here's the fast recap. I ran malwarebytes which removed a few things... removed anything "yahoo" in the about config, and in the default search... then cleared browser data like cookies and history... removed ALL shorcuts to Firefox... and made new shortcuts from the "original" firefox.

Massive thanks to all of you who'd helped me out so quickly!

That was very good work. Well Done. Please flag your last post as Solved Problem so others will know.