Firefox 27 on Mac OS X Mavericks. In a new tab, I open “about:memory”. Show memory reports → I click the button Measure. Firefox gives me the result, with this red warning : “WARNING: the following values are negative or unreasonably large.   • gfx-surface-quartz This indicates a defect in one or more memory reporters. The invalid values are highlighted.” Indeed, I have this in the result : “-0.12 MB ── gfx-surface-quartz [?!]” All I may have done to deserve this ;-) is having many windows and tabs open. Thank you for fixing this. Nicolas