From MozillaZine Knowledge Base

If you are unable to download or save files in the browser and you receive an error similar to, "<filename> could not be saved, because the source file could not be read." try these solutions:

Firefox

This error can occur if the history.dat file is set to "read-only" [1][2]. Open the Firefox profile folder, check the properties of the "history.dat" file and remove the "read-only" attribute, if set.

Mozilla Suite or SeaMonkey

The Download Manager may be disabled completely and a "source file could not be read" error may occur when attempting to download or save a file (bug 180672). Try opening about:plugins, as simply viewing the list of installed plugins may resolve the issue. If that doesn't work, completely exit the application and delete the file "compreg.dat", located in the installation directory\components folder (it will be regenerated). [3].