hello msueirro, sites opening on their own is usually a sign that adware/malware is already active on your system - so please perform all these steps:

1. reset firefox (this will keep your bookmarks and passwords)
2. afterwards go to the firefox menu ≡ > addons > extensions and in case there are still extensions listed there, disable them.
3. finally run a full scan of your system with different security tools like the free version of malwarebytes and adwcleaner to make sure that adware isn't present in other places of your system as well.

Troubleshoot Firefox issues caused by malware

So far this is working great! Thanks for your help. I'd run a full system scan in Safe mode, but it didn't find anything, but your two products found numerous issues and fixed them. Terrific.

msueirro said

When my browser sits idle I will come back to find a new tab has opened without any action on my part. Almost always this tab includes some sort of link. When I touch that link my security suite blocks the action. It is a continuing series of these tabs, all of which are hacking attempts. In addition, I find that when I start my computer Firefox will often launch spontaneously and go to Bing.com as a start page. I don't use Bing at all, so this is another unwanted launching of a window or tab. These things are happening without my consent, and are totally undesirable. There should be a way to prevent the software from opening without any action on my part, or it should be my choice to permit tabs to be opened without my consent. I've searched my computer thoroughly and can find no hint of virus or malware, nor any add-ons that would enable this sort of activity. I'm looking either for help, or for you people to recognize that your software may have a security flaw that is being exploited, one that you need to isolate and resolve.

security tools like the free version of malwarebytes and adwcleaner These do NOT work