This is a really an odd one and it affects both Chrome and Firefox (possibly Safari as well), but NOT Internet Explorer. If you have had unexplained crashes while/after visiting a WordPress (WP) site, it is possible it could be caused by this bug. The good news is this bug has been reported to Mozilla [Bug 1174811] and it was patched on Wednesday (June 17th). Just not sure yet when it is going to be pushed out. Firefox 39 is due out in less than two weeks (June 30th), but then there could also be a 38.0.6 release between now and then (though I haven’t seen anything to hint of such a release).
My understanding is this bug is not malicious, just an annoyance as the browser is secretly trying to download an insanely large file 1 GB TAR file which cause the browser to run out of memory, become unresponsive and eventually crash. Though as an administrator for multiple WP sites I am concerned what exactly in contained in this TAR file. If the main WP configuration PHP file is accessible that could be a major security issue.
Source: The Windows Club