I am wondering if that is possible to set up Brazilian Portuguese as the default language for the firefox addon, everytime I need to ckeck a text it comes up with Portuguese from Portugual or even English.
That’s possible in the add-on configuration, where you can set a default language for the languages that have more than one variant in LT: English, German, and Portuguese. However, there’s currently a known bug which might prevent the configuration from getting saved. We have fixed this but it takes Mozilla weeks to review the new version, so we just have to wait.