Firefox 3 uses SQLite in order to store cookies, downloads, applications local storage, search history, etc.
Unfortunately, updating those databases adds holes. Files keep growing over time. It means waste of storage space and Firefox gets slower and slower.
The webappsstore.sqlite, places.sqlite and urlclassifier3.sqlite files can especially grow very large.
Some extensions like the Digg one also store data in a SQLite database.
In order to optimize these databases, here’s what you should really do from time to time.
Close Firefox and run:
cd ~/.mozilla/firefox
find . -name '*.sqlite' -print -exec sh -c "echo 'vacuum;' | exec sqlite3 {}" \;
If you are on MacOS X, use:
cd /Users/j/Library/Application Support/Firefox/Profiles
find . -name '*.sqlite' -print -exec sh -c "echo 'vacuum;' | exec sqlite3 {}" \;
Of course you need SQLite to get installed on your system.