Normally, if I type e.g. "sup moz" into the awesomebar, a drop-down appears with suggestions for sites and searches etc. But after using the browser for some time (hours, perhaps), the drop-down stops appearing. I still get the "top" completion of the domain inside the actual url bar (completion shown as text with blue background), but not the drop-down. Restarting Firefox sets it right again, but that's quite annoying to have to do all the time.

I think this started on version 56 or 57. I'm using Xubuntu 17.10.