Is there a pattern to when this occurs, for example, when you first start Firefox, any time you go to your home page (or open a new window using Ctrl+n), or at random times?

You could check a few things:

(1) Home page

Firefox allows multiple tabs for home page(s). You can check for the extra page in the Options dialog. See: Startup, home page, tabs, and download settings.

(2) Add-ons

Unexpected behavior may be caused by unwanted extensions. Try disabling ALL nonessential or unrecognized extensions on the Add-ons page. Either:

• Ctrl+Shift+a
• orange Firefox button (or Tools menu) > Add-ons

In the left column, click Extensions. Then, if in doubt, disable.

Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Any difference?

In addition under the options, tabs, I checked "don't load tabs until selected". Three days now and no invasion.