The display of the web page and the operation of Firefox might become slow while using it a long period because of the expansion of the database though the bookmark and the history, etc. are managed by the database in Firefox.
Optimization : in the initial state is only REINDEX. When "VACUUM before REINDEX" is selected, it comes to do VACCUM and REINDEX.
– When an extra space of the data base is less than 20%, only REINDEX is done even if it was changed to the setting that does REINDEX after VACUUM.
Whenever ending 10 times, Firefox is optimized when "REINDEX sqlite database on exit" is selected.
When there are a lot of frequencies of the reactivation of a browser and you want to change timing
Please change the value of sqlite_opt.auto_reindex_per_browser_close from about:config.