Why I left
I stopped using Firefox in the late of November 2016, when Mozilla decided to pull the plug on, and severely restricted Firefox’s add-on platform, in favour of their WebExtensions API.
This issue with this was that the WebExtensions API was relatively high-level - meaning that my favourite extensions (ie DownThemAll!) would cease to work, as they relied on low level access.

Not being able to use DownThemAll! was a game changer for me - it was hands down the secret weapon under the sleeves of my Firefox vs Chrome war.

But that being said, when Mozilla decided to go a-wall, that was the end of Firefox for me

Extensions
Luckily, the developers of most of the extensions that I used in Google Chrome also created the same extension for Firefox - meaning that the porting process was a piece of cake. (Namely Momentum, uBlock, DarkReader, and Wappalyzer

Holding on
In terms of file and document sharing, I use Google Drive a lot.
Unfortunately, Firefox doesn’t allow me to upload folders without… Java…

There are also a few websites that just don’t want to work on Firefox…

New players
I had been using an awesome Chrome extension called SessionBox, which allowed me to run websites under different sessions (credentials, cookies, etc). But sometimes I run into errors when opening Outlook and Google Drive.
In June 2016, the nightly builds of Firefox started including something called Containers, which was practically the same thing.
Not sure if it syncs from your Mozilla account though (EDIT: Don’t think it does)

Firefox’s Web Inspector tool also appears to be more feature-rich than that of Google Chrome, with event listeners attached to the DOM elements.

That being said, I’ll still be keeping Google Chrome on my computer, as with Microsoft Edge and Internet Explorer - yay for web browser compatibility…