I have some JIRA users that are experiencing behavior where after editing/commenting on an Issue, the JIRA menu bar at the top of the screen disappears.
When I say menu bar I mean this:

The page renders such that the menu bar does not appear anywhere on the page. Here is a screenshot snippet of what they see.

From the looks of this, it appears as if the resulting page has rendered on top of the menu bar. You can see this in the top right corner of the screenshot.
We are using JIRA 5.1 and so far this behavior is intermittent. I've asked what specific tasks the users were performing when this occurred. The answers vary from editing the description of an existing issue to commenting on an issue. Also browser usage is latest versions of Firefox or Chrome.
As far as addon-ons go, I do not have any user-installed add-ons that affect themeing:

I checked for errors in atlassian-jira.log, but I don't see anything that stands out as a problem.
Has anyone experienced this behavior before? Any thoughts or suggestions you have would be much appreciated.
Thank you.
-- Stephen