Why do you need Safe Mode?

Could you possibly use a new clean profile instead without installed extensions for testing?

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

You can add -no-remote to the command line to open another Firefox instance with its own profile and run multiple Firefox instances simultaneously.

• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile
• https://developer.mozilla.org/Mozilla/Multiple_Firefox_Profiles

I don't need to get into the various reasons why I wish to start this in SafeMode. I simply want to know how to suppress the prompt. How do I suppress the prompt?

By suppressing the prompt, you will also disable Safe Mode altogether   .......  and that's not what you want, is it  ?

In   'about:config'   -   preference  :   'toolkit.startup.max_resumed_crashes' :   you can set the value to   -1 (minus one)  

Are you starting Firefox with the -safe-mode command line switch?

• https://developer.mozilla.org/Command_Line_Options

You can't suppress that prompt when you start Firefox that way. You can look at the Safe Mode startup code in this file to see how it works:

• https://dxr.mozilla.org/mozilla-release/source/browser/base/content/safeMode.xul