I'd first check to make sure you don't have any unrecognized add-ons installed: https://support.mozilla.org/en-US/kb/disable-or-remove-add-ons

Though it sounds like you might have a malware infection on your system. Try installing this and runing a scan: https://www.malwarebytes.org/antimalware/

See: https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Is there any pattern to the unwanted pages, i.e., a particular site?

Sometimes new windows are externally triggered, for example, using a scheduled task that loads URLs in your default browser. As a test, you could make a different browser your default to see whether that makes any difference (for example, start IE, have IE make itself the default, then continue to use Firefox normally to see whether it makes a difference).

• http://windows.microsoft.com/en-us/internet-explorer/make-ie-default-browser
• Once you're done testing: Make Firefox your default browser

What are on these unwanted pages, ads?

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.

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