This sounds off by an order of magnitude? Firefox's local translation models are only 20-70MB per language pair direction (e.g. en-to-fr or fr-to-en).[1] These models are also only released when they reach at least -5% of Google Translate's COMET score.[1] Currently Firefox ships with support for 32 xx-to-en language pairs and 29 en-to-xx language pairs.[1] As the number of language pairs increases, it probably isn't unreasonable for browsers to stop bundling every language pair and instead prompt users to download uncommon models the first time the user wants to use them.

[1] https://mozilla.github.io/translations/firefox-models/

It only installs models that explicitly downloaded via this API, it seems.

Also, it says to have 22 GB free, but below (under "Note"...), it says the model takes "around a couple of GB".