Is it part of the Google page?

Make sure you are not blocking content.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

One issue that seems more common lately is Firefox's Tracking Protection feature. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature: Tracking Protection {web link}

duststorm said

When click on icon, get usual group of blocks from Google to choose from, but no email block to choose email.

Hello duststorm,

'Not sure what you mean by "blocks from Google" (?)

At startup Firefox opens your home page - maybe you could check the settings  ?

https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

Mozilla Firefox eventually did whatever they do and, so far, the issue seems at least temporarily resolved. I appreciate all the effort.

 All you people are far more digitally, for want of a better word, informed than I, and you  use terms that a less knowledgeable person doesn't comprehend. Like "start in safe mode"; I know what that is, but it has never appeared on this computer, so I couldn't access it. "Blocking content"?. 
 I truly appreciate all the attempts to inform and assist.

@ duststorm,

You marked my post as 'Chosen Solution', but I get the feeling that my post is not what solved your problem .... (?)

In that case : would you please remove that mark from my post ('Undo' button to the right of that post) and mark your own last post as Chosen Solution  ?

Just to make sure : can you access your email account now the way you used to ?