This post is composed within ScribeFire, the blogging extension thing for Frirefox (et al). And I can’t edit posts easily because using the arrow keys causes a blank
A workaround is to edit in plain mode. Solution to be posted if I find one.
Update: Turns out trying to post a video to a tumblr blog froze up ScribeFire real good. This doesn’t help with the original problem, it’s just more fuel for the (Scribe)Fire.
Update 2: Arrow keys work in rich text editor when I install ScribeFire as an addon for my Portable Firefox install (for Windows, arrow key troubles are under Linux). There are certainly a few issues raised on the posting for the last release, dated November 2011 (!)