Firefox gets slower by the time. Lots of data is stored in sqlite databases. First step is cleaning cache – Menu>Tools>Clear Private Data – check Cache and Cookies. Other slowing stuff is the browsing history. Cleaning the whole history is not very effective way so why not remove only the oldest items? Run following in the Firefox’s profile directory:

MONTH=`python -c "import time; print int(time.time()-3600*24*30)*1000000"`
echo "DELETE FROM moz_historyvisits WHERE visit_date < $MONTH;" | sqlite3 places.sqlite
for z in *.sqlite; do echo "VACUUM;" | sqlite3 "$z"; done

This will remove items older than 30 days and maintain all the sqlite databases. Speedup is often very striking.