User Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:42.0) Gecko/20100101 Firefox/42.0 Build ID: 20151029151421 Steps to reproduce: Open multiple(3+) tabs while making multiple calls to a webapi. The webapi call seems to be unrelated to the problem because the crash occurs randomly on different calls. Actual results: The browser crashed. All tabs are frozen. Crash reports: https://crash-stats.mozilla.com/report/index/59324e08-645a-4f64-b7a4-7b4982150923#metadata https://crash-stats.mozilla.com/report/index/f2550cf8-266c-47c9-b1d8-5e85a2150922 https://crash-stats.mozilla.com/report/index/455a7b33-a85a-4600-97ff-250f92150921 Expected results: The page should have been displayed after the webapi call. The errors occurs on different calls each of those are tested and working properly in other browsers.

scaled_font seems to be null there. Can cairo_get_scaled_font(cr) return null in gfxContext::GetRoundOffsetsToPixels?

It's not supposed to, according to the cairo docs: http://cairographics.org/manual/cairo-text.html#cairo-get-scaled-font

Bernd, can you reproduce this consistently? That would be a huge help if you can. Could you attach the contents of the about:supports page?

By the way, the crash reports you listed are all for FF40 and happened in September. Have you experienced a crash recently?

John, The crash occurs at different moments when accessing our website. Only the testers (who use the older version of Firefox) have this problem. This makes it hard/impossible for us to reproduce. I think we can assume that the error is somehow fixed in the current version of Firefox.