It’s for privacy purposes. An online translator requires that all the text you’re reading be sent to a third party, which may or may not use it for nefarious purposes. E.g. maybe you translate your bank account’s web page because there’s a word you don’t know, and now Google knows how much money you have in your bank account.

If you don’t care about that kind of privacy, then there’s no reason you couldn’t use an existing online translator. Firefox has always supported that.