From MozillaZine Knowledge Base
When images are loaded, they can be cached so they don't need to be decoded or uncompressed to be redisplayed. For e-mail and newsgroups (i.e., Thunderbird and SeaMonkey), messages for IMAP accounts are cached as well in either disk or memory cache, unless synchronized locally already. This reduces the amount of network activity to reload previously viewed messages. This preference controls the maximum amount of memory to use for caching decoded images, messages, and chrome items (application user interface elements).
Possible values and their effects
Automatically decide the maximum memory to use to cache decoded images, messages, and chrome based on the total amount of RAM. (Default in all but Thunderbird versions 3.1 or earlier and Minimo).
In Firefox 1.5 and SeaMonkey 1.0 and earlier versions, these defaults were the following:
See bug 296538 for details.
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.
Users who are having problems and want to lower memory consumption can try setting the integer value to something lower than the memory cache setting given in the above table. 4096, which was the previous default value, is a good place to start because it will decrease memory usage for users with more than 64 MB RAM. Users who have no problems with memory should leave the setting at -1.
First checked in
Has an effect in