If you've downloaded it, close Firefox, and then open it again. Come back to FR. Find a post that you want to reply to, click the "Reply" button, and then when your normal Reply box appears, right-click in it with your mouse...
You'll have a list of options. Choose "Open Xinha Here."
The HTML editor box will pop up.
Type your post. Use your mouse and the menu bar to add colors, change fonts, bold, etc.
When you're done, click "OK" and your post with HTML codes will automatically appear in the box.
Click Preview as usual.
If you like what you see, post it. If not, you can right-click in the box again, open Xinha again, and modify the comment before posting it.
Lather. Rinse. Repeat.
Thank you very much.