Chad Scott 110000E1UB Visits (7819)
I ran across an issue today in which MP4 videos like this one won't play in Firefox when previewed in Files. The issue has a long and somewhat complicated history, but here is the TL;DR version:
For the second point, the desktop Windows variants seem to have the decoders installed out of the box. The server variants require installing the Desktop Experience feature. An additional update is required for 2008 R2.
As a bonus, I found the following web tools invaluable for helping to diagnose this issue: