MySpell replaced with HunSpell
The current spellchecker in all Mozilla products are based on MySpell. But MySpell hasn't been developed for a long time and has been replaced by HunSpell. OpenOffice.org which also used the MySpell spelling engine, switched to using HunSpell with it's 2.0.2 version.
Now Mozilla is very close to making the switch to HunSpell. The HunSpell spelling engine has been checked into Mozilla. This means better much spelling features. Read the bug report for more information.