Multiple Firefox browser tabs load  

That looks like some malware or other nasty causing problems, you've ran Malwarebytes and it hasn't found anything. Maybe try something that takes care of smaller items, give SuperAntiSpyware a try, download the free edition Here check for updates before running it.

Those are tracking cookies from advertisers and likely not root of issue.

In your description you said it happens randomly when you are using outlook 365. Is this when you click on a link that it does this?

How many home pages or start pages do you have set in Firefox?

Well it is shot in the dark but 63.0 is out.

Try updating please.

Start or home pages are the pages FF should start when the browser is opened.

Click on the three bars, upper right, then Options, then Home on the left. Top of page on right is Homepages. This is where you set.

Notice the "Use Current Pages". This assumes before you entered Options you have set tab to desired web pages. So lets say on first tab you set you Mail, then on second tab your twitter account. You would then enter Options, Home, Use Current. Every time FF opens it opens with these two pages.

I wondering if there is not some corruption here.

It is not recommended to have to many home pages as it bogs down your start.

Those may be harmless and not a root cause. In my experience not all things found by SaS 'designated' as harmless cookies are harmless. I've seen small unexplained glitches cleaned up by this program.

After deleting all those items was the issue fixed? If that cleared the issue then one or more of those items was the cause of the problem. It doesn't hurt to periodically run it and remove those, there are things that sneak in that will do exactly what you describe, not enough for an anti virus to see, but still enough to cause a problem(s)

Are you still having issue?