This extension allows you to make spellcheck work on whole page body.
What is SpellCheckAnywhere?
SpellCheckAnywhere is a powerful Firefox add-on that enables spellcheck functionality on the entire webpage. With the ability to toggle spellcheck for each browser tab, this extension ensures accurate spelling suggestions, boosting your writing efficiency and accuracy.
To use this, you need to have a dictionary (not language) installed. Technically the Add-on does add the "spellcheck" and "contenteditable" HTML-Attribute to the body tag. The Add-on can toggle these attributes for each browser tab.
SpellCheckAnywhere may not be safe to use and it requires some risky permissions. Exercise caution when installing this add-on. Review carefully before installing.
SpellCheckAnywhere has earned a fairly good reputation and likely can be trusted.