I was running Firefox 64 on Linux Mint 18.2 and everything was file: when I clicked a link for download (i.e a PDF, a XLSX, or some other file) the popup asking how to deal the file (Open with ..., Save file ....) was shown, I made the choice and everything was set in a matter of seconds. After a certain date, FF suddenly stopped to be so reactive: it took some time to display the file dialog and, once shown, it was non reactive at all. I was not able to choose to Open or to Save, proceed or cancel the operation. Simply the pop-up was stuck for a long time. And in the meantime the FF process was taking 100% on one CPU core power. Only after minutes I could be able to go on. I tried to clear the cache, use a new profile or refresh an already used one. Finally, I installed Linux Mint 19.1 from scratch on a different partition (so, not an update) and tried again: no help. Any advice I very welcome. I think I cannot continue this way.