1. Firefox has Spellchecker for more than 100 languages.
Please see Firefox: How do I use the Firefox spell checker?
https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker#w_adding-dictionaries
2. You have to enable ImTranslator in private windows using Tools/add-ons/Imtranslator/Manage
Please see Firefox: Extensions in Private Browsing
https://support.mozilla.org/en-US/kb/extensions-private-browsing