This is an old error message relating to corrupted downloads (http://kb.mozillazine.org/Source_file_could_not_be_read), but has become more prevalent in Firefox 33.

To make a long story short... before, if the server sent a response that was smaller than the stated size, the discrepancy was ignored and the download was treated as okay. Starting in Firefox 33, a file smaller than the stated size is treated as corrupted.

This comment in the bug tracking system has more information: https://bugzilla.mozilla.org/show_bug.cgi?id=1083090#c8

If you have a few minutes for a test... what happens if you stop Firefox from telling the web server that you will accept compressed files? Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste enco and pause while the list is filtered

(3) Double-click the network.http.accept-encoding preference and

(A) If it has the default value (line is not bolded), delete all of the text and click OK.

(B) If it has a customized value, copy the current value to a safe place for potential later use, then delete all of the text and click OK.

When you visit the site again, Firefox should omit the usual headers saying that it accepts gzip/deflate encoded responses. Any difference?