Source file could not be read
From MozillaZine Knowledge Base
When attempting to download and save a file in the browser, you may see an error message similar to the following:
This error can occur if the connection to the server was interrupted during the download. Make sure you are connected to the internet, then try the download again. If the problem persists, try these other solutions:
Mozilla Suite or SeaMonkey 1.x
The Download Manager may be disabled completely and a <filename> could not be saved, because the 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). .
This error can occur after pausing and then resuming a download. Try downloading large files in a single step, without pausing. (It is possible that your security software is scanning and locking the file, which may still be in the system temporary folder after you pause the download. If Firefox can't access the file then resuming the download will fail.) 
If you are using a Firefox extension to manage your downloads, try disabling the extension via "Tools -> Add-ons -> Extensions". 
Note: The source file could not be read error occurred in older Firefox versions if the history.dat file was set to "read-only" (bug 315020).  The "history.dat" file is no longer used in Firefox 3.