@glyph Most recently was because Apple Pay stopped working in Firefox on macOS (after working fine for a couple years), along with being tempted by some nifty-looking Safari extensions that—because iOS jail—would actually work on both the desktop and the phone browser, while my Firefox extensions only work on the desktop.
In the background, I've had growing concerns about Mozilla Foundation being stupid, doing/saying shitty things around crypto/ai/etc, along with repeated rollout of features I absolutely do not want in my browser, that were enabled by default, or the concern that Mozilla itself might be turning into enablers of various sorts and me losing trust in the foundation to follow its mission. Their most recent CEO announcement is adding an awful lot to that feeling.