Right now we use a down arrow/carat in the "background tab" button to indicate tapability and, to a lesser degree, that you have background tabs.
I think that number of background tabs would do a better job of getting the latter message across, and will also be a more useful indicator of what the background tab button does (right now, it could be any sort of menu, based on the arrow glyph).
This will also make the increment/decrement animation more prominent.
Will be fixed as a part Bug 698095.
fixed by bug 698095
Verified fix on Mozilla/5.0 (Android; Linux armv7l; rv:10.0a1) Gecko/20111112 Firefox/10.0a1 Fennec/10.0a1. numbers are increasing and decreasing based on add/removing tabs.