Since my MacBook Pro got a new mainboard, and being told that the new Intel CPUs are not much faster (though more power efficient), I decided to upgrade it with 16Gb of 1600MHz DDR3 1.35v memory, for $181.56 from MSY. This old 2011 Sandy Bridge can handle those specs but Apple does not admit it; so far so good though, as my readings of the internet suggested it would be.
Addendum: Mac OS X leaks like a sieve. The purge command reclaims the "inactive" memory that the OS refuses to via automatic means. Looks like I was wrong to blame FireFox for being the fat pig that it is; there's a fatter one hiding behind it. After a purge the system typically has about 10Gb free.