I've been using AdGuard AdBlocker for quite a while now, and I'm happy to update my previous review.

The only problem I've encountered is a rather unusual one. If my computer crashes, the extension sometimes resets all of its settings to their defaults. Even more strangely, simply importing my previously exported settings doesn't permanently fix the issue. Everything looks correct until Firefox is restarted, after which the settings revert to their defaults again. The only reliable solution I've found is to reinstall the extension first and then import my saved settings.

Fortunately, this is a very rare situation, and under normal circumstances AdGuard works excellently. It blocks ads reliably, has plenty of useful options, and has become one of the extensions I simply can't imagine browsing without.