Do you have the same issue with Firefox in Troubleshoot Mode?

I was able to attach a file. see screenshot What security software are you running?

I use Avast Premium. I'll try Troubleshoot Mode.

Disable Avast, reboot, and try again.

You can check the connection settings in case you use a Proxy or VPN service.

• Settings -> General -> Network: Connection -> Settings

If you do not need to use a Proxy to connect to internet, select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See also:

• https://support.mozilla.org/en-US/kb/connection-settings-firefox
• https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors

You can try these steps in case of issues with webpages:

You can reload webpage(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Settings), click the padlock icon (Clear cookies and site data) for the domain in the selected tab.

"Remove the Cookies" for websites that cause problems:

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Settings -> Privacy & Security
  Cookies and Site Data -> Clear Data -> [X] Cached Web Content -> Clear

• https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox
• https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check for issues with Enhanced Tracking Protection. Firefox shows a purple shield instead of a gray shield at the left end of the location/address bar in case Enhanced Tracking Protection is blocking content.

• click the shield icon for more detail and possibly disable the protection

You can check the Web Console for relevant-looking messages like blocked content.

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/total-cookie-protection-and-website-breakage-faq
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

Thank you both very much for you suggestions. The problem is resolved, but there still seems to be a problem with Firefox.

I tried to disable Avast, but there didn't seem to be a convenient way to do that, so I deleted Firefox instead. I downloaded the standard FF for desktop several times and made different selections each time, and each time it ended up importing all the same cookies and cache and other settings, even though I had deleted everything I could multiple times using FF History. Every time I did that the problem persisted - I could not get out of Google Account Help no matter what I did. So finally I downloaded Firefox Nightly instead, and since it did not import the old stuff I was able to get to Gmail.

This morning (I had to go out last night) I tried to attach a file to an email - the problem that started the whole fiasco in the first place - and it worked fine. So for reasons I cannot explain, the regular FF desktop prevented me from attaching files to Gmail emails and would not allow me to move from Google Account Help to Gmail. Nightly resolved the Gmail issue, but I have no intention of trying to duplicate the Account Help problem because that was just too much of a mess to try to fix again. It also appears that Avast was not the problem.