It works fine on every other site, just on SO it recently stopped working.
I just get the page and the regular orange bar when I'm expecting the white overlay bar to tell me I'm logged in and to refresh the page. Like I said, it works normally on every site except SO.
The error console reports an error when I load the page:
$("#topbar a[href^='/users/recent/']:first").attr("href") is undefined
Included is a picture of it not working, along with the error console.
Edit: after closing out of and reopening Firefox, the problem went away. Any reason why this happened?