There is a small gap in the Subject/Sender/Date/etc. bar (sorry, I don't know what else to call it) in the new default theme on Thunderbird for Mac OS X. Its position changes periodically when the drop- down list menu at the right of the bar is clicked on. I will upload a screenshot to help since I know my definition is vague since I don't remember the correct terms. :-)
This bug is not specific to the thread tree, and it is not specific to Pinstripe. I can reproduce the problem in any Mozilla application using any theme. Firefox's Bookmarks Manager for instance. If you slowly resize the window and watch the column headings, a 1px gap will open and close. Possible dupe of Bug 195692.
Thunderbird no longer looks like this.