I’m having similar problems on my Mac using both Firefox and a clean version of Safari (complete reinstall of Safari with cleared cache…’cause I almost never use it).
It seems the problem is version 4.8.2, as my friend (whose blog I guest write on) is still at version 4.7 something, and I have none of those problems using his admin dashboard on my Mac.
Is there I way I can UN-update and so back to a previous version of WP for my own blog? If not, I do hope these bugs can be fixed quickly, and the workarounds are annoying.