There are a couple built-in ways you can translate, all of them fully local. First is the full page translation and another that allows you to translate a selection (from the context menu.) Fully local nature, of course, reduces the translation capabilities, but I found the quality to be good enough to very rarely need an external service.

Support for in-place website translations has been introduced in FF 118, initially for a handful of western languages, now 50 languages. There is also support for translating highlighted text and user text with about:translations.

It's fully local, which means the translation might not be as good as what Google can do, but it's good enough to use, and in my experience it's usually faster, as there is no network latency (except for the first translation between a language pair, which takes a little while to download the weights).

Really good. I use it frequently and haven't had any reason to switch to an external offering. It can translate the whole page or just a snippet that you highlight.