Could you check whether:

• Yahoo! will load into the tab if you either:
  • Click the Home button on the toolbar, or
  • Press the Alt+Home keyboard shortcut to go to your home page

• Firefox loads Yahoo! automatically in a new window, either:
  • Ctrl+n
  • "3-bar" menu button > New Window

If the problem is limited to the first tab at startup, this has been reported by a few other users but I have not seen an explanation for the problem.

In these three threads, for example, users solved the problem by renaming or removing the prefs.js file, which often contains dozens of settings, and we didn't learn which particular setting was causing the issue.

• Firefox open with new tab not default home page
• When I open Firefox it does not go to my home page even though my setting are set to go to home page when Firefox is opened.
• I want my homepage to be https://startpage.com/. Firefox will not start up with this as my homepage no matter what I do.

I don't know whether you would want to jump right to that or wait for further suggestions. I guess it depends on how many times a day you restart Firefox and have to deal with this annoyance.

Apparently there is an issue when Firefox is set to clear history at shutdown and doesn't finish the job, it may resume at startup and run into a failure that prevents Firefox 44 from showing the home page. That issue is likely to be fixed in Firefox 45, scheduled for release next month.

To try to ensure Firefox completes the job at shutdown, it's best to exit from the menu, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit

If you typically close Firefox using the "red X button" on the title bar or other methods where Windows may kill Firefox prematurely, please give that a try and see whether it makes any difference.