Custom CSS (user stylesheet) to set your own emoji for twitter's "like" icon. Replace "content" emoji with your own choice.

heartbreaker.css

/* Developed from http://www.shortlist.com/tech/gadgets/heres-how-to-replace-twitters-heart-with-your-own-emoji 
Copy / paste your own emojis into the content: lines below
Safari: Preferences -> advanced -> Style Sheet
Other browsers: https://userstyles.org/
*/

.HeartAnimationContainer {
visibility: hidden;
}
.HeartAnimationContainer:after {
/* content: '✩'; */ /* general case, un-comment this and remove/comment those below if you just want one emoji for liked / unliked */
visibility: visible;
display: block;
position: absolute;
}

.ProfileTweet-actionButton .HeartAnimationContainer:after {
content: '★'; /* Not liked by viewer */
}

.ProfileTweet-actionButtonUndo .HeartAnimationContainer:after {
content: '⭐️'; /* Liked by viewer */
}