This is a fantastically useful little extension.
Can be Found on the Mozilla Addons Site here.
Go tools/open profile folder
that simple !!
This extension is best for looking around.
It is possible of course to edit files, but as Firefox is still running there are of course still limits as to what you can do.
For major changes, I suppose it is still best to edit in Windows with Firefox closed. But for most jobs this is not necessary.
Still you don't want to cut off the branch you are sitting on !
Enjoy !
It is worth noting that for Example Fast Dial has a Fast Dial Folder in the Profile.
But if you go to the Extensions folder in the Profile, there is another Fast Dial Folder.
Similarly for Simple Mail