I recently learned the developer of Spellbound, the best spell checker ever for forms in Firefox 1.0, went to work for Mozilla to incorporate the extension to Firefox 2.0. Therefore there was never a compatible release for 1.5. But I found a little known forum project that has an extension that seems to work just as well.
Before installing the Spellbound 1.5 Dev extension you’ll need to install a Mozilla language dictionary. If you already had Spellbound installed before upgrading to Firefox 1.5 your existing dictionary will work.
Mozilla Dev Language Dictionaries
Just save this file to your desktop, open Tools, Extensions from the Firefox 1.5 menu and drop the downloaded file into the Extensions window. Restart Firefox.
Install Spellbound Dev 1.5