If you are as regular a Firefox user as I. You do need to be aware of two key learnings about Firefox extensions.
First, it’s not a good idea to load up your browser with multiple extensions. The greater the installed extensions, the longer the browser takes to fire up (load into memory for the non-geeky). And think by installing then disabling extensions speeds matters up. You are W-R-O-N-G!
As far as Firefox is concerned those extensions must be loaded. Before the next instruction is to disable them. They remain embedded into memory along with the rest of the browser. Until you exit. And even then, some remain sort of sticky.
Part two of this brief posting is to list the extensions with known memory leaks. Read all about them at the MozillaZine Problematic Extension list.