I mostly use Firefox, except I use Chrome for certain tasks at work [1]. There is one thing about Firefox that occasionally has me on the verge of ditching it for Chrome.

It has a terrible spell checker. I find about as many errors in its spelling as it finds in mine.

There used to be one other thing that really irked me, but it looks like they recently fixed that (I'm on the beta channel, so not sure if everyone has this yet). That was handling of keyboard shortcuts. Shortcuts, such as OPTION-HOME to go to the home page, did not work if the focus was in the address bar, the "search in page" field, or a text entry field on the page.

[1] Chrome's dev tools seem more responsive and better organized, and it handles multiple profiles better.