I am yousing Fluther now on a pc with Firefox and, as u can c, i don’t think there is a spell checker build in in Fluther.
In Opera (browser) i have a checker installed, and it helps me avoid (most) typos.
A spell-checker is at work on Fluther. If it is not Fluther’s it must be Firefox’s. There is a certain eclectic vocabulary in the offerings that make me think it is Fluther.
But only the Fluther gurus know for sure. @rebbel don’t you get the red lines under the not-too-correct words?