Why does Firefox's memory usage jumps up dramatically when it's exiting?
Firefox just now was using around 140MB. But as soon as I told it to exit, the memory usage jumped to 350MB before it began going down and finally quitting.
With Virtual Memory systems there is no simple measure of memory. For example you could be looking at working set. This is the memory that is actively (or at least recently) in use by the application. Memory that was allocated but has not recently been used (e.g. for an extension that has not been referenced by any action) will tend to be paged out to make room for memory that can be actively used.
When an application shuts down it tends to make use of code and memory that is otherwise not used, likely including memory that has not been referenced since start up (e.g. shutdown code for extensions was parsed at startup but then not used until shutdown).