I added a Performance entry for fx65 with a link to the Task Manager - see what tabs or extensions are slowing down Firefox article.
From my post July 24, 2019 in that article's discussion thread, https://support.mozilla.org/en-US/kb/task-manager-tabs-or-extensions-are-slowing-firefox/discuss/8045
<snip>Also, there's no "Background" thread for this article. The Firefox 65 Discussion forum thread just links to this article. So does the Firefox 65 release notes. The bug reports I found (such as Bug 1478832 [meta] New about:performance experience) weren't that helpful. I tried a web search but the best I came up with for an overview are third-party articles: https://www.ghacks.net/2019/01/09/firefox-65-task-manager-with-memory-readings/ https://www.ghacks.net/2018/10/11/this-is-firefoxs-upcoming-aboutperformance-page-huge-improvements/ https://techdows.com/2018/11/mozilla-makes-about-performance-as-complete-task-manager-in-firefox-65.html https://techdows.com/2018/10/mozilla-overhauls-firefox-about-performance-page.html