Again: no they aren't, except in a narrow and strategically irrelevant "well it's still a missile you can launch".
If you're executing a first strike, then your goal has to be to take out your opponents ability to retaliate...or you're trying to pull off a limited first strike.
In both cases, submarines add no strategic value if used: in the limited strike scenario you give away a submarines location, and in the first-strike scenario you are not going to have enough missiles, with up to date enough targeting data, in order to hit targets any quicker then land-launched ICBMs are.
The time you waste trying to maneuver your submarines close enough to your target to launch, the fact that it's entirely possible they get taken out before they launch their entire payload (US and Russian subs shadow each other as a matter of course to track their movements), and the inherent communications problems coordinating with submarines all make them irrelevant as first-strike weapons.
The point of a nuclear ballistic missile submarine is a guaranteed second-strike capability. Your opponent might surprise you take out your bombers. They might surprise you can take out your land-based ICBMs. But there's no way they can ever be sure they got every boomer sub, and boomer subs don't target military targets: they target cities, capitals and command bunkers. They exist to ensure no useful advantage is ever gained from first-strike.