Feature ideas get a bit lost here in the support forum. You can submit suggestions in one or more of the following ways:

(1) Feedback site: https://input.mozilla.org/feedback

(2) Developer mailing lists (sometimes hard to figure out which one is best): https://lists.mozilla.org/listinfo

(3) Bug tracking system (yours would be a request for enhancement): https://bugzilla.mozilla.org/ and https://developer.mozilla.org/docs/Mozilla/QA/Bug_writing_guidelines

I don't think that such a feature would be possible on Windows XP as there is only one global volume setting available. On more recent Windows platforms this may already be possible more or less by using multiple Firefox instances with a separate profile.

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