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Cannot install web developer because: Invalid file hash (possible download corruption) -261
I cannot access my web developer tasks anywhere. I tried to install again but got this message: because: Invalid file hash (possible download corruption) -261
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Try to enable third-party cookies temporarily during the download and installation of the extension. --- Enhanced Tracking Protection in Firefox for desktop Third-party cookies and Firefox tracking protection
Since 3.6.6 I've been having the same problem. Not just one extension but on several.
Save the xpi file to your desktop and install the extension by dragging the file in a Firefox tab or use File > Open File. See also http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
I'd checked everything from that website without success. Things worked fine with 3.6.4. It seemed to have broken with 3.6.6. Would a re-install help??