Finally I’ve found enough time to write a new extension for Firefox that fits my needs
I use Thunderbird as email client but also as rss feeder, I prefer Thunderbird because when I read new emails immediately I can see new contents on my subscribed feeds.
Adding feeds to Thunderbird is easy but if you come from a browser navigation you must get the feed url from page and then type it on Thunderbird.
I prefer to have feeds on clipboard and paste them on Thunderbird!
RichFeedButton makes exactly this! Copy your feeds on clipboard, period!