HAR (HTTP Archive) files can be used for a variety of performance analysis techniques, and also for debugging. HAR files can be attached to a bug raised during testing - the developer then can run the HAR data locally to find out what went wrong.

We can export our network tab timeline to HAR from the Chrome Developer Tools by right clicking on the network output and seleving "Save as HAR with Content".

Firebug requires the installation of another add-on called Net-export, created by the lead Firebug developer and the creator of the HAR format.

chromeHAR is an online tool that provides the same timeline interface as Chrome and can be used for this purpose.