I have a brand-new, fresh instance of Firefox Portable 55.0.3 running on a Windows 10 Pro machine. All antivirus is disabled for testing.
Steps to repro:
1. Visit a site like:
2. Start playback of the media file and quickly click to skip ahead/around the media.
3. Watch as Firefox Portable 55.0.3 tabs crash (and sometimes the browser itself crashes).
I have been seeing this behavior on a variety of media-focused sites since the 55.0.3 update occurred. These tab/browser crashes do not occur in the desktop version of Firefox 55.0.3 on the same machine, only the Portable version.