How to add Rmail as a choice in Firefox 2.0

First of all open Firefox configuration panel typing about:config in the URL field.

In the filter field type contentHandlers to load the default values.
Probably, you will see a few lines like these. The total number of these lines should be a multiple of 3.

As you can see for each aggregator there are 3 strings.
The feed type (browser.contentHandlers.#.type, it should be application/vnd.mozilla.maybe.feed), the feed reader URI (browser.contentHandlers.#.uri) and the name (browser.contentHandlers.#.title).

Let's add Rmail.
Since Rmail is my 4th reader I will need to replace # with the index 3.
Please note that indexes start from 0.

Here there are the steps.

Right click > New > String > Name (browser.contentHandlers.types.3.title) Value (Rmail)
Right click > New > String > Name (browser.contentHandlers.types.3.type) Value (application/vnd.mozilla.maybe.feed)

Before setting the last string let's give a look at how Rmail add new subscriptions.
If you load on your browser the URL http://www.r-mail.org with the right rss and mailto arguments you can add a new feed.

Example
http://www.r-mail.org/?rss=http://www.simonecarletti.com/blog/index.xml&mailto=noreplay@noreply.net
This string will create a new subscription sending all new items from my feed to the specified email address.

Now let's set this string as a Firefox subscription URI.

Right click > New > String > Name (browser.contentHandlers.types.3.uri) Value (http://www.r-mail.org/?rss=%s&mailto=YOUR_EMAIL).
Instead the feed URI use %s to tell Firefox to replace the label with the current feed URI.

Don't forget to replace YOUR_EMAIL with your email.

Now restart Firefox 2.0, open a feed (for example mine) and Choose a Feed Reader.
Select the radio field Use a web site and choose Rmail.

Confirm and click subscribe to subscript the feed using Rmail.

Start from the beginning to add new web aggregators.