Can I repair the cracks between the bricks in my fireplace?
Sure, you can re-cement those. You'll want to dig them out as much as you can, rinse them out with water, let dry, spray with water to wet them &amp; then force in the new cement.

You can put a batch in a Zip-Lock type bag &amp; cut off a tiny bottom corner of the bag to squirt the mortar into the joints. Below, is what the Home Improvement stores offer. The Bucket is good for up to 3000°F &amp; tubes are good for up to 2000°F.