I can imagine how frustrating it would feel as a Firefox dev reading reports like this and wondering wth is going wrong. Because I'm "just a user" and I've never seen FF being slow to start or loading pages.
Same with sync, I recently install Firefox on a new laptop. Clicked the sync button, logged in with my account and done. In 2-3 minutes all history and addons sync'd \o/ UX wise I really love that Firefox Android has the address bar on the bottom :D
Except some Google stuff like the GCP console.
EDIT: formatting