Add-ons • Re: Translating French web pages back to English

You understood correctly that the language packs (and dictionaries) available for Pale Moon are for the situation opposite to yours: they are for people to have the browser translated into French (in the user interface) and have French spell-checking.

What you want, instead, is a translator extension. I'm not sure if there are any currently-maintained ones, other community members may be able to point you in the right direction. Otherwise using Google Translate or similar web-based translation services and feeding them the French page URLs might be a solution.