Because of the pivot from XUL and forgoing of the firefox extension model for a copy that is exclusively Chromium's.
The announcement of XUL removal started a time of uncertainty for community developers that led to increase in addons bugs or being abandoned, that webextensions still struggle to match in quantity and quality
Together with constant UI changes, tab groups removal, occasional serious bug, bookmark bugs, firefox sync bugs, and in general being slower, just created a too unstable environment. And people were slowly forced to look at alternatives, forks or chrome like, palemoon, waterfox, icecat, librewolf, chromium, iridium, ungoogled-chromium, brave, opera...
I am still thinking in changing back to Firefox, as it is better with x2go which I started to use recently, but my main day to day is still in Iridium until I get time to change it