When I open a new Firefox window on developer edition 48.0a2 and click on the downwards-facing arrow to open awesomebar history, I just get a tiny blank window instead of a list. This persists for a solid 10 seconds or more before repeated clicks finally get it to work as it should. http://i.imgur.com/eSJX4iO.png

It may be a problem with an add-on or theme. Can you reproduce this in Safe Mode? https://support.mozilla.org/kb/troubleshoot-firefox-issues-using-safe-mode

Yep, same issue in safe mode.

It doesn't seem to happen with a blank profile, so I'm wondering if there's some perf issue with the way the new awesomebar is populated relative to the old one, might make this only reproducible with old, heavily populated histories or something.

would you mind attaching a log from about:support? It may also be interesting if, after a backup of places.sqlite) you could try to run this add-on https://addons.mozilla.org/en-US/firefox/addon/places-maintenance/ and report if it helps with the perf and the statistics it reports. Is the slowdown only happening when you click the dropdown arrow, or also when you actually type something in the locationbar? After it happens, are the next openings fast or do they keep being slow? In the next days I will uplift a perf fix to Aurora, that will resolve a problem with histories having a large number of very big urls, and maybe that will help, I will ask you to retest when it lands.

Sounds like related to bug 1279864

is this still a problem?

no answer and no other reports, so I guess we may consider this working.