Make sure that Firefox closes properly to avoid opening a possibly crashed session on the next start.
Try to use "Firefox > Exit Firefox" to close the Firefox application and the currently open tabs and windows.

See "Hang at exit":

• http://kb.mozillazine.org/Firefox_hangs

See "Firefox hangs when you quit it":

• https://support.mozilla.com/kb/Firefox+hangs

I tried "Create a new places database" from https://support.mozilla.com/kb/Firefox+hangs. Unfortunately this did not fix the problem. The only Start-up Option which is working correctly is Options/General "Show a blank page." "Show my home page" and "Show my windows and tabs from last time" both give the same results, although it's not my windows and tabs from last time. It's the tabs from when I first noticed the problem regardless of what I have opened since.

This problem is being manifested on two separate Windows 7 64-bit machines and two instances of Ubuntu via VMWare emulation. These installations are all synced, so I wonder if that could be a factor. There is also a synced Mac laptop which is starting correctly in showing the start page.

I fixed it. I noticed that even when starting from a blank page, when I went to my actual home page (by clicking the Home Page button), the same four offending tabs would appear. I cleared the Options/General Home Page URL, replaced it with the Mozilla default and restarted Firefox. Then I manually went to my home page and told Firefox to Use Current Page for the Home Page. Then I restarted Firefox again. That worked.

Thanks for your response.