If you’re also cursing Firefox 56’s total system hangs, and want to downgrade to the previous version of the Web browser: Firefox 55.0.3 Win64 En-GB. Yes, I tried the suggested remedies. Renaming the files places.sqlite and cert8.db to *.old and then letting Firefox replace them with fresh ones on restart. Didn’t work. I’m now very seriously looking at switching to Opera, rather than having to go to Firefox 57 and be a guinea-pig for experimental code and suffer the looming “won’t run legacy add-ons” problem. Nearly all my Addons currently sport the yellow “Legacy” banner.