Browser.sessionhistory.max total viewers
From MozillaZine Knowledge Base
Pages that were recently visited are stored in memory in such a way that they don't have to be re-parsed (this is different from the memory cache). This improves performance when pressing Back and Forward.
This preference limits the maximum number of pages stored in memory.
Possible values and their effects
Automatically determine the maximum amount of pages to store in memory based on the total amount of RAM (Default)
Do not store any pages in memory.
Any positive integer
Maximum number of pages to store in memory.
Users who are having problems with memory consumption can try setting the value to something lower than specified in the table above to lower memory consumption. Users who have no problems with memory should leave it at -1.
First checked in
Has an effect in
See Using Firefox 1.5 caching for developer details on this feature.