DAMN Firefox's stupid "reopen last session" option ! I turned it off, it is now back on !
This morning I had some kind of bizarre meltdown where Firefox kept trying to open new window after new window. Even when there were dozens of windows open, it kept opening more, slowing down as the processes ate up more and more memory. Response to the mouse and keyboard took several minutes. I had to click on the desktop and use Force Quit to get out. But if I try to relaunch Firefox now it starts doing the same thing ! I re-installed Firefox and the problem persists. Does anyone know where the file containing the last session info is kept ? Can I delete it without affecting bookmarks ? Any other hints ?
OOOOOPS ... I know better, but still didn't say I'm running this instance of Firefox under OSX 10.9.
= new reply since forum marked as read
Sounds like you have a virus.
Opt for the FREE version (unless you care to pay), not the trial (pay) version.
https://www.malwarebytes.com/
Best of luck!
Avast never did a thing for me. It sounds like you have a PUP virus IMO. This is what they do. 
I strictly use Malwarebyes and SUPERAntiSpyWare. These two programs cover me nicely and also REMOVE things that should not be on a computer, esp. Malwarebytes.
You can always delete the program if it does not help but it sure sounds like a PUP virus to me -- all of those folders opening up like that = PUP IMO.
...recurrent 'open new...' problem but having hit that a few times, I created a shortcut on my desktop (Win7pro):
"C:_Program Files (x86)_Mozilla Firefox_firefox.exe" www.google.com (replace the underscores with backslashes)
This opens a single window (not the last session) pointed to google. (You can change the web address to anything.)
This works even if the option to open the last session is enabled.
I'm not sure how to do this for an apple OS. 
Response to eppur_se_muova (Original post)