[Fontconfig] Inconsistent state after font upgrade

[Michael Witten]

I upgraded my DejaVu fonts (http://dejavu-fonts.org/) from version
2.32 to 2.33, and I didn't bother to restart any of my programs
because such an action didn't even ocurr to me as being necessary.

However, I think that this upgrade caused an inconsistent state for at
least Firefox 4.0.1, which I assume uses fontconfig to manage fonts.
The result was that at least a few of the variable-width serif
ligatures were being rendered with the wrong glyphs (presumably
because glyph locations had changed due to the upgrade); see here for
a screencast of the problem:

  http://www.youtube.com/watch?v=zZoK4BhZaCs

After running a new instance of Firefox, the problem seemed to be
resolved, corroborating this hypothesis.

Is this a fontconfig weakness that could be resolved?

Sincerely,
Michael Witten