On the surface they're quite similar, I agree. I won't pretend that Firefox has some whole separate UX decision process that's ruining it for me. It is, like all decisions between similar products, a matter of my own pet nit.

I dislike Firefox's zooming behavior. I have to zoom into websites because I have poor vision. Chrome's zoom behavior has been extremely natural for me, and easy to adjust to. When I used Firefox I attempted to use 'vanilla' as well as an extension that aimed to improve the zooming behavior by separating scale of text from scale of other elements, etc.

I was unable to find a solution that worked for me. I hated using the extension, which itself had a pretty bad UX, and I couldn't get preferences to save properly for individual webpages.

I can't use a browser with poor zooming behavior, I rely on it too much. As I write this I am zoomed in 250% in Chrome, for example.

Those pet-bugs or pet-nits are, in my opinion, the huge thing that keeps people from moving (alongside the perceived friction of moving data over). I wouldn't read too much into mine.