There's three main things I've noticed. These things only started happening after Mozilla's change in leadership. While they have announced more features since then, I feel like the quality control has gone down a bit.

1. Even with the sidebar disabled, the sidebar will show on the side of the screen for half a second after launching Firefox
2. Sometimes, the "x" buttons on tabs will stop working. I have to middle-click the window to close it or close that window and open a new one
3. The AI popup often gets in the way when selecting text. I think it would be nicer to have it show up in the right click menu after selecting text. I disabled the AI so it won't show up since I never really used it anyways.