Firefox 72以降で動作。7つの定数値を書き替え後、ブラウザーコンソールにコピペして実行。

履歴のURL置換.js

(async function () {
const FAVICON_URI = NetUtil.newURI('https://www.pixiv.net/favicon.ico');
const PRINCIPAL = Services.scriptSecurityManager.createContentPrincipal(NetUtil.newURI('https://www.pixiv.net/artworks/'), {});
const OLD_URL_SEARCH_TERMS = 'https://www.pixiv.net/member_illust.php?mode=medium&illust_id=';
const OLD_URL_PATTERN = /^https:\/\/www\.pixiv\.net\/member_illust.php\?mode=medium&illust_id=[0-9]+$/;
const NEW_URL_SEARCH_TERMS = 'https://www.pixiv.net/artworks/';
const NEW_URL_PATTERN = /^https:\/\/www\.pixiv\.net\/artworks\//;
const REPLACE_PATTERN = [/\/member_illust\.php\?mode=medium&illust_id=/, '/artworks/'];

const query = PlacesUtils.history.getNewQuery();
const options = PlacesUtils.history.getNewQueryOptions();
options.includeHidden = true;

const newURLs = [];
query.searchTerms = NEW_URL_SEARCH_TERMS;
let root = PlacesUtils.history.executeQuery(query, options).root;
root.containerOpen = true;
for (let i = 0, l = root.childCount; i < l; i++) {
	const url = root.getChild(i).uri;
	if (NEW_URL_PATTERN.test(url)) {
		newURLs.push(url);
	}
}
root.containerOpen = false;

const oldHistoryItems = [];
query.searchTerms = OLD_URL_SEARCH_TERMS;
root = PlacesUtils.history.executeQuery(query, options).root;
root.containerOpen = true;
for (let i = 0, l = root.childCount; i < l; i++) {
	const node = root.getChild(i);
	const url = node.uri;
	if (OLD_URL_PATTERN.test(url)) {
		oldHistoryItems.push({
			url,
			title: node.title,
			lastVisitTime: node.time,
		});
	}
}
root.containerOpen = false;

let i = 0;
for (const oldHistoryItem of oldHistoryItems) {
	if (i % 100 === 0) {
		console.info(`${i}件 ⁄ ${oldHistoryItems.length}件`);
	}
	const newURL = oldHistoryItem.url.replace(...REPLACE_PATTERN);
	if (!newURLs.includes(newURL)) {
		await PlacesUtils.history.insert({
			title: oldHistoryItem.title,
			url: newURL,
			visits: [{transition: PlacesUtils.history.TRANSITION_LINK, date: PlacesUtils.toDate(oldHistoryItem.lastVisitTime)}],
		});
		PlacesUtils.favicons.setAndFetchFaviconForPage(
			NetUtil.newURI(newURL),
			FAVICON_URI,
			false,
			PlacesUtils.favicons.FAVICON_LOAD_NON_PRIVATE,
			null,
			PRINCIPAL
		);
	}
	await PlacesUtils.history.remove(oldHistoryItem.url);
	i++;
}
console.info(`${oldHistoryItems.length}件の履歴を置き換えました`);
})();