I have added the Spell Checker 1.2 add on but my messages in my email browser are not checked for spelling, or if they are, the mistakes are not highlighted or identified in any way.
How can I enable this simple tool?
A spell-checker certainly comes already on in v3+, and I'm pretty sure it was there in v2. What version of FF are you running?
Should show up in Tools > Add-Ins, btw.