Just a moment ago, I wanted to install some Firefox extensions and update some others, but every time I tried FF gave this error:
“Firefox could not install the file because: Unexpected installation error. Review the error Console log for more detail. -203″
The fix has been quite easy. First result on a Google search was this page, where they explain a method that seems quite a drag (basically, whipe all the config out and reinstall everything), but some guy suggests in a comment that the only file you need to remove is extensions.rdf. It worked for me, just by deleting that file the problem was gone. 
By the way, on MacOS X this file is in:
$HOME/Library/Application Support/Firefox/Profiles/PROFILE/extensions.rdf.