Hello. I have the following proposal: the extension of compact Firefox databases to compact the file urlclassifier3.sqlite. This file in my opinion requires regular compacting. Today I did it using SQLite Manager, which reduced the size this file from 60MB to 21MB.
You're correct about having to keep up with compacting those SQLite databases in Firefox as they grow very quickly. I use this line of code inside in Firefox's Error Console often to vacuum it, however it doesn't tell you how much smaller it is after the vacuuming.