As it currently stands on firefox mobile (for the ipad at least), if you have a significant number of tabs open and close out of the one currently selected, the browser will jump so that a further away tab, is now the one selected.
For example, if I open tabs A through V ( with A-L visible in the tab bar when A is the "first" and current tab):
and then close tab A:
I'm now on tab S (with H-S visible in the tab bar).
This makes it frustrating to navigate after closing a tab. It would be must less confusing and more convenient for "B" to always become the new current tab after closing "A".