This is a continuation of issue https://support.mozilla.org/en-US/questions/1223605?fpa=1, which is closed. That thread is fairly recent (September 2018, I believe), and I am having this same problem with Firefox on Windows 7 consuming all memory in an 8GB machine. One of the solutions proposed there was "about:memory 'Minimize memory usage'". I've done that and received the response "Memory minimization completed (2018-12-28T22:10:01.798Z)". My (initial) question is this: does this function PERMANENTLY minimize memory usage, or just for that session, or even just for that instance and it will then continue to consume memory after clicking the 'Minimize memory usage' button? If it's only instantaneous or for the session, this does not solve the problem. I concur with one of the posters in the referenced thread that I have other programs to run and after Firefox consumes all memory it can take several minutes simply to open a Task Manager windows, let alone open a spreadsheet; and can take 5-10 minutes to log off. This seems like a recent phenomenon with the 64.0 update.