RogerCD said

I could not get past the master screen (I'm not sure what it's actually called.)

What is your computer system and Firefox?

The page displayed when Firefox is started is called the homepage. What are you using as the home page?

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. Is the problem still there?

I am responding to myself in hopes that someone will read this and have some advice. I was just this minute stupid enough to bite Mozilla's insistence that I download the same update -- and of course, the same thing happened. Someone was kind enough to respond to my first question and I was going to write back to him, but I can't get into my email. When I open Firefox 38.5.1 in Windows 8.1 it opens on the home page and will not let me access inbox or messages sent or messages deleted or contacts or anything for that matter. It's like it's frozen on that page. Thank You, Roger

Again replying to myself as a means of adding information as I cannot get into Firefox. I closed Firefox to attempt to access it in safe mode as suggested, but now Firefox will not open at all.

If you can't get Firefox to work, use Win IE for now.

Fred, I am so sorry. I "misspoke." The references to Firefox should have been Thunderbird. If you or anyone has any suggestions regarding the Thunderbird update of today causing the problems mentioned above, I would appreciate any advice.

Thank you, Roger

I've called the big guys to help you. Good luck.