After Firefox 63.0.1, instead of releasing 63.0.2, Mozilla released Firefox 63.0.3 with bug fixes and crashes. Here are the issues that are resolved in version 63.0.3 according to the release notes.
Unity Games using WebGL gets stuck in Firefox and the browser gives the message “A web page is slowing down your browser. What would you like to do’. Mozilla identified and fixed the problem.
When Firefox is configured to automatically detect proxy settings, the browser sends multiple requests and causes performance issues, which is not the case when Internet Explorer or Chrome is configured to automatically detect proxy settings. The Firefox team found the cause and fixed the problem.
Due to videos getting stuck in background tabs, the company has disabled HTTP response throttling in this release.
This release fixes the issue where opening magnet links does not work.