[frenchyatwork]

IIRC, Firefox doesn't always give back memory when the machine has free memory. If you start running other high memory processes, then Firefox might dump some memory. The reason for this is 1) there's not much point in freeing memory if it will be unused and 2) switching web pages is something browsers do a lot, and freeing and then reallocating large chunks of memory is a significant performance hit.

[Ostrogodsky]

That's fantastic in theory, not so much in practice. Visual studio code crawls into a halt and the only way to "get it back" it is restarting Firefox, even if it had just one tab opened.