A regression was found in Firefox and Thunderbird: check spelling isn’t functioning.
If you install a myspell dictionary (myspell-en-us is installed by default), it cannot be used in Firefox or Thunderbird.
To remedy this, run these commands in terminal:
sudo ln -sf /usr/share/hunspell /opt/firefox/dictionaries
sudo rm -rv /opt/thunderbird/dictionaries
sudo ln -sf /usr/share/hunspell /opt/thunderbird/dictionaries
Then open Firefox, go to a page with a form, right-click in a text field, and make sure you selected “Check spelling”, and that you selected the appropriate language:
Now, when you type something in a form, the text will be automatically checked for spelling.