>>58713
A remux is not a transcode, the original video bitstream stays the same, only the container changes, so yeah, you may safely delete the original (so long as you don't have any attachments to the qt file format).
>>58714
What you are referring to by "conversion" is a transcode where you re-encode the original bitstream using a different codec, which results in a new bitstream. This obviously affects data and costs quite a bit of your computing resources. The remux, on the other hand, costs practically nothing as you are not encoding anything. (lastly, the firefox thing is mostly a microshaft winblows fuckup. hevc files played fine on firefox linux for a long time)