penders - 1:10 am on Oct 21, 2013 (gmt 0)
Actually, don't change an existing handler! Read on...
Then mailto.2 and mailto.3 are something to do with Chrome
This isn't "Chrome" as in "Google Chrome" (the browser) in case that is what you are implying? This is "chrome" as in the browser GUI. I would guess that 2 and 3 are simply placeholders.
Looking more into this, it would seem that you need to increment the value of gecko.handlerService.defaultHandlersVersion for Firefox to process any changes.
Type the following into the address bar (which is actually the default value for mailto.2 and 3):
From that page you will see:
# increment this number when anything gets changed in the list below. This will
# cause Firefox to re-read these prefs and inject any new handlers into the
# profile database. Note that "new" is defined as "has a different URL"; this
# means that it's not possible to update the name of existing handler, so
# don't make any spelling errors here.
What that last bit means (and is worthy of caution) is that if you change the .name of an existing entry and change the defaultHandlersVersion then it will create an entirely new entry in the profile database, in addition to the entry you thought you were overwriting - it won't overwrite the previous handler if you change the name.
I changed mailto.2 and it seems to work OK.