The strength of Firefox lies in its plugin architecture which is leagues ahead of the competition. Adblock on Firefox is superior to other browsers because it can block dynamic content/video ads. Noscript, Requestpolicy, sharemenot, refcontrol, betterprivacy, certificate patrol, and on and on. These are all addons that I use that either have no alternative in other browsers due to technical limitations or have alternatives that are lacking in functionality. Firefox is also the only major browser left that is 100% open source, most of Chrome is based on Chromium but there is still proprietary code in there and there isn't really a conveniently packaged Chromium version for production use.
There are a lot of things I like about each browser and I rotate browsers frequently but I often find myself relying on Firefox for the plugins it has that are a step above the rest. I am not really a fan of the new number versioning scheme but it does seem like improvements are happening faster than before, which can only be a good thing for the end user.