If you have previously set browser.urlbar.unifiedcomplete to FALSE in order to disable this messy, annoying "Visit <site>" prompt whenever you type something in the URL bar, do yourself a favor and do NOT update. This preference is now ignored and FF forces an ugly, wide "awesomebar" where the URL to the right of the page title instead of below. Thanks a lot, Mozilla devs! Good on you for forcing something literally all advanced users hate and ask for an option to remove.
THAT is what killing Firefox - developers IGNORING actual user requests (how many years since people asked them to Sync custom search engines users add?) and adding stupid "trendy" junk, not it lagging on someone's machine for years because of something they still can't set up right. (Doesn't lag for me even on a Core M) Firefox succeeds because it's a browser for advanced users who like to customize their experience to streamline their workload. It will quickly lose to Chrome if it tries to become Chrome, because Chrome is better at being Chrome. You know the saying: don't try to argue with a fool, they'll drag you down and beat you with experience.