Open File dialog has the OK button grayed out

This article also applies to other problems you may be having with the Open File dialog, which is the one that pops up when you click on a link to download a file that prompts you to open the file with an application or to save it to your hard drive. The OK button being grayed out is documented in bug 264492. If you are experiencing that bug or other weirdness, try renaming the mimeTypes.rdf file in your profile folder as it is usually caused by a corrupt mimeTypes.rdf file.

Note that there is a workaround to manually edit mimeTypes.rdf, but you should only attempt it if you feel particularly daring and have thoroughly customized your File Types settings (which is the information mimeTypes.rdf contains).