Is your FF viet language version?

No it's english version

That sometimes will happen EN grammar checks will not correspond correctly to the language used other then the EN FF version.

Then I should switch it to Vietnamese version of it?

HI, please detail what you are running as a Dictionary and Spell Check If not having one or both you possible should install and test. https://addons.mozilla.org/en-US/firefox/search/?platform=windows&q=Vietnamese

Please let us know if this solved your issue or if need further assistance.