http://kb.mozillazine.org/Application_not_responding

http://kb.mozillazine.org/Compacting_folders

Tools → Options → Advanced → Network & Disk Space → “Compact folders when it will save over X MB”

Preferences → Advanced → General → Advanced Configuration → Enable Global Search and Indexer

Account Settings → (each account) → Junk Settings

Preferences → Security → Junk

Reset Training Data can fix some problems and is global affecting all accounts.