I will give you a list of every single Fire type that can reach over UU thanks to their defensive merit:
Heatran
Arceus-Fire
If this is not a good enough of an indicator for you on the quality of fire as a defensive typing, the answer is no.
As long as you get good second typing, Fire is good defensive typing. And more important - keep SR off the field.
Looking for examples ?
For example Moltres is really, really interesting typing when it goes to resistances and immunities. Also his bulk is more then solid with 90/90/85 with great ability Pressure, so you may in fact tank those Stone Edges or other low PP SE hits with Roost/Substitute. Let's check them:
x4 Grass
x2 Fire
x2 Fighting
x2 Steel
x2 Fairy
x4 Bug
Immune to Earth
Look at those. Some of those are phenomenal resistances on special side (Fire, Fairy, Grass), while his typing allow him to check some physical attackers as well (Scizor, Hippowdon, Fighting Types) as long as you keep SR out of the field. I'm 100% sure Moltres would be OU material in previous Gen, but... SR screw him up and Rapid Spin/Magic Bounce weren't the most reliable ways to keep rocks off the field (unlike Defog now). Add to this that he has reliable recovery (Roost) and phazing moves actually makes him good wall/tank.
And with Defog buff no matter how you guys say it, but keeping SR off the field is much easier to do, helping Moltres alot. So yeah, Moltres has defensive merit, just keep SR out of field. Of course Ho-Oh is better at everything now, but... I somehow doubt he'll stay with us for too long.
Rotom-H is another example of really interesting defensive Fire type. This time his resistances:
x2 Fire
x2 Electric
x4 Steel
x2 Fairy
x2 Bug
x2 Flying
x2 Steel
x2 Grass
x2 Ice
Immune to Earth
Holy friggin cow. Look at amount of those resistances. 9 resistances, one Immunity and TWO weaknesses in Water and Rock, which are easy to cover (Hint: Empoleon, Poliwrath, Keldeo, Chesnaught, Virizion, etc. etc.). While his lack of reliable recovery kind of suck, RestTalk is certainly more viable this Generation while invested and as a tank he's still fine as well. Again - looks one of the best defensive typings out there.
Charizard X is actually another interesting option to consider. While of course offensive will still be his main niche, but I wouldn't totally underestimate his defensive capabilities. Resistances:
x4 Fire
x2 Steel
x4 Grass
x2 Electric
x2 Bug
While technically his weaknesses are Rock, Dragon and Earth. While those sound troublesome, there are Pokemon which actually cover all of those (Bronzong is your perfect partner here, Togekiss and Whimsicott cover all except Rock, Skarmory should do the trick as well, Empoleon cover everything except Earth, etc.) while your bulk when invested is actually pretty good and you have high base attackin stats, so you don't really need to invest to hurt stuff back. And access to Roost is nice as well.
There are few other examples, but those prove that Fire types may be strong in defensive way as well. Problem since Gen 4 was always Stealth Rock. But as I mentioned - with Defog it's indeed easier to keep rocks of the field, as it's not blockable like Rapid Spin and it's distribution is pretty good. You want to use defensive fire type ? Run 2 x Defog or Rapid Spin + Defog or Magic Bounce + Defog, etc. etc. In this generation it should be MUCH EASIER to keep rocks away. And looking at how some of them have really, really good defensive potentials, why would you not do it ?