Confirmed on this device (Firefox 33.0 on Linux) and with Firefox 33.0 on Mac OS X Maverics. Without developer tools open, my site loads in <1s. When developer tools are opened, and I navigate to the debug tab for the first time, the browser consumes 100% of one of my cores for ~60s. It eventually completes the processing (sometimes firefox reports a slow script, but if it is allowed to run, it finishes), and performs normally - sourceMaps seem to work fine. On each subsequent page load, the browser behaves as when navigating to the debug tab for the first time. Cursory testing in a recent version of Chromium does not have the same slowness. The site contains a fairly large amount of javacript, which is processed by browserify as part of our build process. Browserify is responsible for generating the sourceMaps.