Moderators, please move this topic to http://forums.macrumors.com/forums/site-and-forum-feedback.66/ if the crashing is not specific to pre-release Sierra. I tried twice to upload a file, both attempts led to a crash of Adobe Flash Player – reported within Firefox, but clicking for more details failed to present details. The crashes occurred before a file selection dialogue could appear. Not reproducible with Safari. The player is up-to-date.