See bug 296538 for details. 0 Do not cache decoded images and chrome in memory. Any positive integer Maximum amount of memory in KB to use to cache decoded images and chrome (1 MB = 1024 KB). (Thunderbird default: 4096. Minimo default: 256.) Thunderbird 3.3 will switch to the "-1" default per bug 629247. Caveats browser.cache.memory.enable must be true for this preference to take effect. Lowering