Your startup setting might have changed. You normally get that warning if you have Firefox set to start with a home page or a blank page. You normally do NOT get the warning if you have Firefox set to restore your previous session tabs and windows automatically.

You can check that here:

• Windows: "3-bar" menu button (or Tools menu) > Options
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

Near the top of the page, you should find the Startup section with the "When Firefox starts" options.

Note that in order for your previous session to be restored, you have to avoid enabling either:

• Automatic private browsing
• Clearing history when Firefox closes

If you want to double-check those, while you're still on the Options/Preferences page, click Privacy & Security in the left column.

On the right side, the History selector can have one of three values:

• Firefox will: Remember history -- you're good

• Firefox will: Never remember history -- NOT compatible with restoring previous sessions

• Firefox will: Use custom settings for history -- check the details:
  • "Always use private browsing mode" NOT selected
  • "Remember my browsing and download history" selected
  • "Clear history when Firefox closes" NOT selected --or-- if this is selected, click the "Settings" button to the right and make sure that you are not clearing "Browsing and download history"

Can you get it working the way you like?

Doh - fighting with Win10 and its insistence on everything especially Edge/Bing caused my changing the start page. Thank you.

You can set your start page otherwise you allowed it when you used bing to change it.