The Wordzen Add-On for Firefox has been officially approved by the Firefox Add-On Site. It took longer than Chrome approval, because Firefox requires a human editor to review all browser add-on submissions. Given what we do though, we are big fans of human review.
The official Firefox Wordzen listing is here:
https://addons.mozilla.org/en-US/firefox/addon/wordzen/
You can now install the Wordzen plugin for Gmail in Firefox with confidence, and have your emails proofread and edited.