ublock origin is still available in the Firefox add-on store.

The developer has pulled the 'lite' version, which is developed mainly for Chrome because Google killed some APIs the full version was using.

You can continue to use Ublock Origin, which uses the v2 manifest.

The delisted extension, Ublock Origin lite, is a v3 manifest plugin. Apparently it was created to address chrome blocking the v2 extension, but you can continue to use the v2 extension on Firefox