One of the features I think is unmatched in other browsers is Firefox's ability to search open tabs on your other devices, and also to almost instantly send a tab to another device. I mainly use it from mobile to desktop (when I stumble on a site that I feel may be risky to let it run all scripts on a mobile phone).
That is a very useful feature! FYI both Chrome and Safari have their own implementation of that, though it requires using the vendor's login system (Gmail; iCloud).
