I keep accidentally clicking the star icon in my favorites and un-favorite-ing folders when I am just trying to open them up! Sometimes I click on the numbers for the new messages, which is where the favorite star is hidden and I end up clicking the star instead. Perhaps place the favorite button star in a place where it isn't so easy to accidentally click, especially if it is going to be hidden until one hovers over it with their mouse.
I do really like the new folder panel and favorite folders, this is the only problem I'm having.
Agreed, the favorites button takes up ~20% of the total button making it easy to hit accidentally - especially in a touchscreen environment where you take your finger from the right and try to touch the first available area on the folder button. I do not know how often most people mark a folder as a favorite (which I may be entirely antiquated as most tech-savvy users keyword search an email), but I think that is must be low enough to rate a right click, or even hide it in a settings tree.
Andreas Friesen commented
They should place the favorite star button on the left side.