Rated 5 out of 5 stars

After a frustrating 4 failed attempts, I've got it to work.
1. Right click on the "Add To Firefox" button
2. Choose "Open link in new window". If necessary, give permission to open. A software installation dialogue box appears.
3. Click "Install now"
4. Restart Firefox
5. Go to "Tools - Options - General" to ensure that Spellcheck is ticked
6. Go to some multiline message input box on a Firefox tab . Right click. Confirm that "Check Spelling" is ticked. Choose "Languages - English - United Kingdom".

And it now works. At least on my laptop it does.

This review is for a previous version of the add-on (1.19).