Here is how –
This will make the whole page text editable, and now the spell errors are easily visible same as when you see it appear while writing mails or something else in firefox. That’s the default spell checker of firefox browser.
Or you better copy this bookmarklet to the bookmark section in the browser, and whenever you want, just click on the bookmarklet and it will act as it should.
Its the easiest way to check for spell errors after you have drafted some post in your blog and need to check for spellings quickly from the preview of the post.