Sorry to hear you're having trouble spell-checking. Perhaps get in touch so we can point you in the right direction of how to enable spell checking in other apps where this is used? No data is shared with anyone - please examine the code to confirm this yourself.

This is not an issue with the spell-checker - this is an issue with your browser settings. In Firefox, go to Setting > General. Under the 'Language' section, make sure to tick 'Check your spelling as you type'. Once that is enabled, you should start seeing in-line form mis-spelling highlighted with red underline.

--UPDATE-- This one is very strange! It works on the freecycle.org form field called 'Post Title' but does not for the field 'Description'. My multi-line check is set to level 2. [about:config >> layout.spellcheckDefault >> set to 2]. I'm going to hazard a guess...this might be the way the form is setup there. I've never seen that before TBH.