While about:memory does describe Resident as:
Memory mapped by the process that is present in physical memory, also known as the resident set size (RSS). This is the best single figure to use when considering the memory resources used by the process, but it depends both on other processes being run and details of the OS kernel and so is best used for comparing the memory usage of a single process at different points in time.
The vsize is also significant. Resident seems somewhat stable while vsize continues to grow until firefox becomes unusable and I end up restoring to killing the process.
Being able to track both Resident and vsize would be helpful!
I would let @amcreight comment on it but from our side we do not want to put any more information into the widget. In the future we will have tooltips which should be able to give additional information.
I would oppose vsize as a metric since there is too much noise with regards to OS, size of addresses and memory installed. If it's useful for Windows users, perhaps we can have it default to 'off' and add the ability to enable it once the panel lands.