This is neither a bug nor an easter egg. From docs:
CTRL+SHIFT+X: Toggle typing direction of the current textbox.
Do you happen to know why the feature requires that the suggestions list be displayed? That fact made it seem more buggish to me, as it seems like an oddly specific trigger.
On the latest (stable, 3.0.10) version of firefox (Vista), I can toggle it without the suggestions list displayed.
That version on XP and Gentoo seems to require the suggestion list to be displayed. I guess the feature is a little ragged, anyway -- I imagine not too many people use it.
After all this time, we've finally found a benefit to using Vista over XP and Linux! Congrats, HN! ;-)
Actually, it does seem there's a bug -- if the right-most character (left-justified) is '/', then it mistakenly prepends that character to the right-justified version.