Is it because you are opening pages that were previously open ?

You may have set many home pages. If not see one of these, then for other causes see

• Firefox repeatedly opens empty tabs or windows after you click on a link
  IIRC the problem with firefox opening many Windows because of the content setting was resolved

Obviously Malware is one possible cause, that may need to be considered.

All but one are blank pages with nothing in them, not even all of my toolbars. I thought of Malware or a virus but a scan does not find one.

Update: I cleared the cache and then went to Tools/Options/Startup and checked the line When Firefox starts "show a blank page". I'm not sure which of these did it but the problem seems to be solved.

It is possible that there is a problem with the files sessionstore.js and sessionstore.bak in the Firefox Profile Folder.

Delete the files sessionstore.js [2] and sessionstore.bak in the Firefox Profile Folder.

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

If you see files sessionstore-##.js with a number in the left part of the name like sessionstore-1.js then delete those as well.
Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost, so you will have to create them again (make a note or bookmark them).

See also:

• [1] http://kb.mozillazine.org/Session_Restore
• [2] http://kb.mozillazine.org/sessionstore.js

Sounds like you have another program running in the background. Google Maps did this to me - it would not close down. To solve this set your home page to a site you would not normally visit - anything will do, then revisit your normal home page and reset it again. Problem should be gone - but I avoid Google maps now because I don't want it to happen again. However Google Earth is fine.

Good luck!