you hit the nail - it was the intention  🙂

If any of the files are modified the hashvalues of the file(s) is changed creating a mismatch against the torrent-file.

Using Firefox will not help unless the problem is viewing media directly from the internet and have the the proper codecs and it's all set up properly.

While having the right codec helps, if the stuff you already has is corrupted, then it won't help.

Also, too many codec is a bad thing, just get what you need. Many times people go crazy and get conflicting codec and that causes the problem.