What is your computer system and Firefox?

When you start the browser, does the icon show up on the computer taskbar?

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?

First, let me correct myself. I think it may have actually been a Firefox update that preceded the problem, not Windows.

ASUS desktop PC CM6730 series. Windows 10 home version 1607, OS build 14393.222. I hope this is the information for which you asked. I don't know how to check Firefox's version but it was the newest one as of a few weeks ago and I imagine it surely is now as I reinstalled it a day or two ago.

If the taskbar is the row of icons below the desktop screen, the icon was always present along with others but when I tried to load it, the blue line underneath indicating the app is open is absent. Since uninstalling and reinstalling, the icon does not appear when I try to open it. Trying to open it while holding the shift key has no different effect that I can detect. Thank you for taking an interest.

Let’s do a full clean re-install; Download Firefox For All languages And Systems {web link} Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86) Mac: Open the "Applications" folder.
https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory.

Do Not remove the profiles folder.

Look for, and rename any Mozilla or Firefox program folders by adding .old to them. Do not rename the Mozilla Thunderbird folder.

After rebooting the computer, run a registry scanner. Then run the installer. If all goes well, remove the OLD folders when you are done.

Can you recommend a safe registry scanner? I'm a bit leery of unknown programs.

Don't worry about it. Some anti-virus programs have one.