I am unable to finish the installation (after around 40%) and there is an error message saying I have to restart the navigator, which I did, as well as the computer.
But when I do then online test of Flash Player Help, it tells me thatFlash Player is actually not installed.
My configuration is MacBook Pro 2013, MacOSX 10.14.3 Mojave, Firefox 74.0.
Thank you for your help.
The 3rd screenshot seems to indicate Flash Player was installed successfully. You can double-check if Flash is indeed installed by going to System Preferences. If you see the Flash Player item listed, it's installed:
Firefox, like all other browser vendors, disables Flash by default, forcing the user to allow it (per session) when it detects Flash is required. The 4th screenshot seems to indicate the browser was not refreshed/restarted after installation. See Firefox's tech note https://support.mozilla.org/kb/why-do-i-have-click-activate-plugins for instructions to enable Flash in the browser.
I am unable to reproduce:
After installing Flash for Firefox and restarting the browser, I navigate to https://helpx.adobe.com/flash-player.html and see the plugin icon to the left of the address field:
Clicking the plugin icon, displays the prompt to Allow Flash:
Click 'Allow' to Allow Firefox to run Flash, then click on the yellow 'Check Now' button: