They might not be able to sack you, but regardless, what do you do if they simply say "no"? Either you end up effectively sacking yourself by taking the new offer, or you put up and shut up and they remember your bluff for the next salary review.
What you should be asking, when you have a career planning type meeting with your boss, is asking "hey boss, what do I need to do to get an awesome raise this year?". That doesn't reveal you're looking around, maintains your aura of being a keen and ambitious member of staff, and puts the ball in their court. The answer could be:
- nothing (eg. no budget or fixed salary levels)
- deliver an awesome piece of work on time/budget
- take on additional responsibilities
- achieve a professional qualification which makes you more valuable
- hope that the company as a whole has an awesome year
and you can then judge whether the effort required and the likelihood of it happening, plus the environment you like, is worth it in preference to moving (and the risk of an environment you don't know). Ultimately all salary negotiation is a quid pro quo - you get more money by being more valuable and less replaceable to the business.