This is a tracking bug for the following Mozilla bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=397608
XUL splitters are inaccessible

In Evolution and other (gtk) paned environments, one can usually press F8 to move to the splitter and then use the arrow keys to move it followed by enter to finish the process.  We can also access and set the value via atspi. 

This is not the case with XUL splitters which can be found in both Thunderbird and Firefox.  And when the splitter does not afford enough room for the text (e.g. mail folders in Thunderbird, bookmarks in the FF side bar) the panes don't seem to become scrollable either.  The text simply gets truncated.  Speech/braille users should still be able to access the text, but folks who rely primarily on magnification will have a harder go of it.