Firefox uses sqlite to store many data. Like bookmarks, cookies, etc. But it doesn't provide a direct option to re-index these data, which is a feature of sqlite3 and eady job. One will need sqlite executable to perform the task yourself.

So in sqlite:

sqlite3 datafile VACUUM;
sqlite3 datafile REINDEX;

Will re-index the datafile and slightly boot firefox performance after a certain period.

In bash with unix utils:

find ~/.mozilla -type f -name "*.sqlite" -exec sqlite3 {} VACUUM \;
find ~/.mozilla -type f -name "*.sqlite" -exec sqlite3 {} REINDEX \;

In windows, which lacks command-line automation. You might need to run sqlite commands one file-by-one. But, with gnuwin32, one can do regular shell jobs in windows cmd.exe:

ls -1 *.sqlite | sed "s/\(.*\)/sqlite3 &1\ VACUUM;/g" > indexer.bat
ls -1 *.sqlite | sed "s/\(.*\)/sqlite3 &1\ REINDEX;/g" >> indexer.bat

This prepares the indexer.bat for your use.