it may be because of a male ware infection ,install malewarebytes and have a scan http://www.malwarebytes.org/mbam.php

Do you get any error or debug screen when this occurs?

Sorry this may not help much. Have you read the information in firefox crashes and followed the troubleshooting steps carefully (please say what you have done) .

Your crash type (from the crash signature) is not at the moment a common cause of crashes, the problem is probably likely to be with your machine and its setup rather than a Firefox Problem affecting many users of the Mac OS.

• try creating and using a new profile see basic troubleshooting_8-make-a-new-profile
• try checking for malware see
  • is my firefox problem a result of malware
  • this general advice (sorry not certain how much of this is applicable to a Mac) /questions/914980#answer-302372

For reference

• Your Report for CrashID e6b21421-f2d5-4974-acb9-b1cc62120408
  • Crash Signature: nsPrefBranch::RemoveObserver
  • Related bug 725632

I note that your modules show no dates in the crash report modules, but maybe that is normal with that type of crash or with a Mac

Create a new profile as a test to check if your current profile is causing the problems.

See "Basic Troubleshooting: Make a new profile":

• https://support.mozilla.org/kb/Basic+Troubleshooting#w_8-make-a-new-profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins" in case there are still problems.

If that new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

I have tried to relaunch firefox in safe mode, but with no success. otherwise have not tried anything else, not knowing what to do other than follow the directions on the troubleshooting website. unable to open the profile in terminal.

I have a mac, no malware support for mac.

Are you able to follow the instructions in https://support.mozilla.org/en-US/kb/Managing-profiles#os=mac&browser=fx11

• Are you able to open the terminal ?
• The procedure is to open the profile manger to create a new additional profile.
• Do you see the option to create a new profile as shown in the attached image from that article ?

Sorry I am not a Mac user and no doubt cor-el will provide better information but the troubleshooting article does seem to suggest forums do advise on security and malware problems on a Mac see https://support.mozilla.org/en-US/kb/Is%20my%20Firefox%20problem%20a%20result%20of%20malware?s=malware&r=0&e=es&as=s#w_how-do-i-get-rid-of-malware

• as part of https://support.mozilla.org/en-US/kb/Is%20my%20Firefox%20problem%20a%20result%20of%20malware?s=malware&r=0&e=es&as=s#os=mac&browser=fx11
  recommending
  • http://www.bleepingcomputer.com/forums
  • http://www.spywarewarrior.com/index.php
  • http://www.spywareinfoforum.com/

It may be of interest to see another couple of recent Crash IDs to see if they are the same signature as the one you have already provided.