Menu option Edit > Preferences > Advanced tab > Option: "Compact all folders when it will save over 20 MB in total" is selected and has been selected since FireFox was installed over a year ago. Today, while researching another unrelated matter, I learned that it is a good idea to weekly invoke menu option File > Compact folders. I had never invoked this menu option so I did so today. After it completed the status bar indicated "Done compacting (approx. 248 MB saved). Today, and everyday since FireFox was installed, have been normal days reading some emails (~5/day), writing some emails (~2/day), and emptying the Junk folder (~10/day). There has been no recent wholesale deletion of emails. If the option to "Compact all folders when it will save over 20 MB in total" has been selected, how is it possible that 248 MB was made available when I manually selected menu option File > Compact folders? I appears to me the automatic compact does not work. Ubuntu 16.04 LTS updated everyday. ThunderBird 52.1.1 (64-bit) (this version appeared yesterday).