Well you could if you really wanted to, but I suspect you might be disappointed. IMO the main cause for the impression of it being slow will probably be the low-power AMD E-450 CPU, slightly up in performance from an Intel Atom, somewhat less than a Celeron or an old Athlon.
Of course there can be other reasons for a system to seem sluggish.
Another factor restricting the potential benefit of more memory is the single channel memory controller, so there will be no increase in bandwidth with a second DIMM fitted. This means you wouldn't get quite as much operational speed increase as you would on a system with dual channel memory controller. However the extra memory can be a solution if the poor performance is being caused by paging to the hard drive, as long as the system is running 64 bit, which you would expect a new system to be doing.
Firefox in my experience can have memory hogging problems, changing to a leaner browser is probably a better option than simply throwing memory at it.
One method of researching the correct memory is to use a supplier who guarantees compatibility, such as Crucial:
or simply Google search the existing memory type:
Edited by Platypus, 31 December 2013 - 08:06 PM.