Firefox: video can't be played because the file is corrupt. Works with Avast off

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0.  Win 10 64bit, latest Firefox, Avast, Flash

1.  Click on 1 of 6 .mp4 video links  on my home page eg http://rnsdb.x10.mx/rnsdb/vcoins-invcoll.mp4 Firefox gives me the above error. 
2.  Clicking on 1 of 5 .mp4 video links in another folder eg http://rnsdb.x10.mx/recs/v1dcp.mp4 works ok
3.  Everything works ok in Chrome and Edge browsers. If files are downloaded or opened, they play okay, as do the video files that were uploaded to my web site
4.  Turning off Avast virus checker fixes the problem, so I don't know if it's Firefox or Avast.
5.   Entering a URL exception in Avast for http://rnsdb.x10.mx/* doesn't help
6.  Clearing history doesn't fix. Adobe flash Shockwave 18.0.0.232 is latest version.
7.   Firefox about:config and changing media.windows-media-foundation.enabled to false doesn't help nor does unchecking "Enable Adobe Flash protected mode"
8.  Sometimes, hitting Firefox reload 6 or 7 times in a row plays the video, but it reverts back to failing next time
9   It's doing the same thing on 3 Win 10 desktops and 3 Win 8/10 laptops all with  latest version software

The strange thing is why one set of .mp4s should play and another set in another folder not play.  I uploaded them several times and it still fails, so the files are not corrupt.  Is this a Firefox problem or an Avast problem?

Thanks, Ron

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I think it's an issue with Avast. In addition to internet connection issue, it's preventing me from finishing downloads of files. I try to download a file from a file host site and it always fails when it's finishing (while Avast's shields are enabled). The file successfully downloads when I disable Avast's shields. I've been having a lot of issues since Avast's latest program update and have to constantly disable it to get things done.

[Milos]

Hello,
thanks for the report. I will redirect it our QA to test and reproduce the issue.

Milos

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I reported this earlier as a bug to Firefox.  They confirmed it and said it would be fixed in the next release of Firefox (41) September 22

Ron