You can create a boolean pref extensions.zotero.debug.memoryInfo and set
it to true, and then when you reopen the prefs there'll be a Memory Info
button at the bottom of the Advanced preferences that shows the Firefox
(It's also possible to connect from the devtools of a Firefox 60 ESR
build to a Zotero source build with the -debugger flag  and use the
Memory tab, but I think that's mostly just a prettier view of the same
data, and considerably harder to set up.)
We fixed a pretty significant memory leak that affected large imports or
long sessions in 18.104.22.168, so you should make sure they're running that.
Windows would only be relevant because we're still using a 32-bit build
there, with a 3 GB memory limit. But obviously Zotero generally
shouldn't be using 3 GB to begin with.