The tabs are draggable: you can reorder them if you wish. So for example by left clicking and holding on the rightmost tab, you can drag it left to its new position. The little arrows and line are insertion points: where the tab will end up if you let go of the button now.
The barred circle indicates that you can't drag a tab into a browser window (or, indeed, almost anywhere else).
If it's doing it while you're not pressing the left mouse button then, as KW says, you've got a sticky mouse button.
Right clicking on a tab lists other options you may not have noticed before, including refresh all tabs, close this tab, and close all tabs except this one.
Incidentally, since you say you create new tabs to drill down through threads, did you know holding down the control key while clicking on a link will open that link in a new tab?