Hmm, I'm not sure where that error is being displayed. If it is a selection of lines from a crash report, could you give the report ID number or a link to the full report on the crash-stats server?

Is there any pattern to getting that crash involving a particular website?

.....

Okay, I found it:

CASecureBrowser9.0 9.0 Crash Report [@ strlen | libsbruntime.dylib@0x3ba5 ] https://crash-stats.mozilla.com/report/index/53ac93cc-846a-4fdd-8f72-e97e20170502

I don't really understand what the CASecureBrowser is (http://ca.browsers.airast.org/). Maybe it is a separate program based on Firefox 45 code? Anyway, I don't think I can help much with this crash. Hopefully they have a support line.