Re-buffering happens if you replay a video after it buffers completely till the end and if you click on the timeline far enough back during buffering.
I’ve looked through many threads but didn’t find any fix/workaround for this. I saw some users saying changing values for media.cache_readahead_limit and media.cache_resume_threshold to 99999 fixed it but that didn’t help and actually caused Firefox to stop buffering midway through videos.
Wouldn’t surprise me if nothing could be done to fix this. It might help if the quality level was fixed instead of variable.