Mozilla has released Firefox 85.0.2 to the stable channel. This minor release fixes a bug that causes the browser to freeze on startup. The bug has existed in Firefox for a long time, but it was version 85 that increased its case frequency.
Firefox 85.0.2 fixes the deadlock that caused it to hang on startup. For example, under a certain set of circumstances, a hang occurs when starting Firefox through a .desktop file in the Xfce environment. It could be reproduced in macOS Big Sur 11.0.1.
It is noted that the deadlock bug has been around for a long time, but changes in Firefox 85 have significantly increased the likelihood of freezing.
The release notes are available here.