Looks like tabbing backwards when using Firefox breaks on some editor blocks. Seems to me it happens just for some block types, not sure if this is already a known issue.

To reproduce, just tab through the blocks and controls till the block that in the content sample content is after the split one ("... like this one, which is separate from the above and right aligned.") then tab backwards.
If the trap doesn't happen the first time, tab again forwards and backwards.

To make it simpler to understand, I've made a video using a very strong focus style:

https://cloudup.com/ckt5HyHe5K7

In the last part of the video, I'm just keeping Shift+Tab pressed and as you can see, as soon as the previous block toolbar appears, focus is moved back to the following block again, in an endless tab sequence loop.
I can't reproduce in Chrome.