We need more exact site that is the cause for others to see if they get the same problem to know what to look for.

It is possible that security software (anti-virus, firewall) is causing the problem. Try to disable security software temporarily to see if that makes a difference.

Only difference was the Firefox version. Everything else was identical. Same AV, same firewall, same pc, same windows 10, same url, same Firefox settings. Nothing changed except the Firefox version. It is something Firefox added in its newer versions.

Are you saying that the video starts from the beginning? Or does it pause then resume during the upload?

Firefox shows the video loading (5%, 20#, ....) till it reaches 100% and the "play" arrow icon appears. But, before there is time to click on that arrow icon to begin playing the video, the downloading buffer begins all over again (5%, 20#, ....). And this repeats so that the video never plays.

This does not happen when a different browser (Chrome, IE) is used or an earlier version of Firefox is used. It is only when using the 1 or 2 latest versions of Firefox Quantum that this happens. Very weird and very annoying.

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?

FredMcD said

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?

I did that before asking. In safe mode the video runs normally. But after returning to normal Firefox the same problem recurs. And, yes, there are no addons or plugins installed. None! Not even disabled One cannot live permanently in safe mode! What additional code does Firefox (recent versions only) turn on when exiting safe mode? That is what is causing the problem. The problem does not happen with earlier versions (even with several addon installed in those earlier versions). So it is something the developers added to native Firefox (by which I mean Firefox just installed with no addons or plugins beyond what came with the fresh install) in the last 1-2 versions. What did they add to make it "better"? Is there a log I can run to see what Firefox is doing?

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes). If the problem goes away, that tells us one thing. If it does not, that tells us something else.

It could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Hi, Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove your Firefox program files and then reinstall Firefox. This process does not remove your Firefox profile data (such as bookmarks and passwords), since that information is stored in a different location.

To do a clean reinstall of Firefox, please follow these steps: Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from this page) and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (or open the Firefox menu and click the close button ).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: 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.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

More information about reinstalling Firefox can be found here.

WARNING: Do not use a third party uninstaller as part of this process. Doing so could permanently delete your Firefox profile data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be easily recovered unless they have been backed up to an external device! See Back up and restore information in Firefox profiles.

Please report back to say if this helped you!

Thank you.

Also please use Windows search to find Device Manager and check if Windows 10 has replaced your Video Card Drivers with Microsoft's. Is so download and install your drivers. https://www.howtogeek.com/302595/how-to-stop-windows-10-from-automatically-updating-hardware-drivers/

FredMcD said

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes). If the problem goes away, that tells us one thing. If it does not, that tells us something else.

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It could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

You are correct. That was the problem. Disabling Hardware Acceleration solved the problem. Thank you.