Import/export tiles for WebExtension version of NewTabTools. Paste into console in new tab, then execute `exportTiles()` and `importTiles()`.

ntt_webext_importexport.js

// blob to base64:
//  https://stackoverflow.com/questions/12786818/file-api-blob-to-json
//  https://stackoverflow.com/a/11562550/3217805
// base64 to blob:
//  https://stackoverflow.com/a/16245768/3217805
var exportTiles = function() {
	Tiles.getAllTiles().then(tiles => {
		return Promise.all(tiles.map(tile => {
			return new Promise(resolve => {
				let fr = new FileReader();
				fr.onload = function (event) {
					tile.imageString = btoa(String.fromCharCode(...new Uint8Array(event.target.result)));
					resolve(tile);
				};
				if (tile.image) {
					fr.readAsArrayBuffer(tile.image);
				} else {
					resolve(tile);
				}
			});
		}));
	}).then(tiles => {
		let exportDiv = document.createElement('div');
		exportDiv.style.cssText = 'position: absolute; width: 100%; height: 100%; z-index: 100; opacity: 0.9; background: #fff; color: #000;';

		let exportDivButton = document.createElement('input');
		exportDivButton.type = 'button';
		exportDivButton.value = 'close';
		exportDivButton.onclick = () => {
			document.body.removeChild(exportDiv);
		}
		exportDiv.appendChild(exportDivButton);

		let exportDivText = document.createElement('p');
		exportDivText.textContent = JSON.stringify(tiles);
		exportDiv.appendChild(exportDivText);

		document.body.appendChild(exportDiv);
	});
};
var importTiles = function() {
	let importDiv = document.createElement('div');
	importDiv.style.cssText = 'position: absolute; width: 100%; height: 100%; z-index: 100; opacity: 0.9; background: #fff; color: #000;';

	let importDivInput = document.createElement('textarea');
	importDivInput.style.cssText = 'width: 90%; height: 80%;';

	let importDivButton = document.createElement('input');
	importDivButton.type = 'button';
	importDivButton.value = 'import';
	importDivButton.onclick = () => {
		let tiles;
		try {
			tiles = JSON.parse(importDivInput.value);
		} catch (error) {
			window.alert('bad JSON format');
			return;
		}
		console.log('PARSED:', tiles);
		return Promise.all(tiles.map(tile => {
			try {
				if (tile.imageString) {
					const byteCharacters = atob(tile.imageString);
					const byteArrays = [];

					for (let offset = 0; offset < byteCharacters.length; offset += 512) {
						const slice = byteCharacters.slice(offset, offset + 512);
						const byteNumbers = new Array(slice.length);
						for (let i = 0; i < slice.length; i++) {
							byteNumbers[i] = slice.charCodeAt(i);
						}

						byteArrays.push(new Uint8Array(byteNumbers));
					}
					tile.image = new Blob(byteArrays, { type: "image/png" });
				}
			} catch (error) {
				tile.imageString = '';
				window.alert('error converting tile image: ' + JSON.stringify(tile) + '\nError: ' + error);
				return;
			}

			return Tiles.putTile(tile).catch(error => {
				tile.imageString = tile.imageString ? '' : tile.imageString;
				window.alert('error importing tile: ' + JSON.stringify(tile) + '\nError: ' + error);
			});
		})).then(() => {
			window.alert('Import OK!');
		});
	}

	let importDivCloseButton = document.createElement('input');
	importDivCloseButton.type = 'button';
	importDivCloseButton.value = 'close';
	importDivCloseButton.onclick = () => {
		document.body.removeChild(importDiv);
	}

	importDiv.appendChild(importDivButton);
	importDiv.appendChild(importDivCloseButton);
	importDiv.appendChild(document.createElement('br'));
	importDiv.appendChild(importDivInput);
	document.body.appendChild(importDiv);
};