1. download WebMail extension and extension add-on for your webmail service.
2. configure your account
3. configure extension to get Junk mail
4. use these instructions for reporting multiple emails

Thank you for posting this. Can you expand on those sections with step by step instructions which will minimize further questions. Specifically line 2 and 3.

Also, if you are related to the http://webmail.mozdev.org/ site, you have a bit of work to do.

There is a typo (POP and SMPT protocols should be POP and SMTP protocols).
And at least from IE, the only links on the site are the two at the bottom. Not even a download link.

While I have no need for this tool (I use IE and spamcop's email service to download from these types of webmail acounts), it could probably be very useful for some. Perhaps you need to do some more work before advertizing it, however.

IE does not support correctly CSS, so it shows page not correctly and you can't see links on top.
I'm not related to site, I only translated extension to my language.

I can't edit my post, so I'll write links here.
1. Download links: http://webmail.mozdev.org/installation.html
2. Account Setup: http://webmail.mozdev.org/setup.html
3. Configuring WebMail http://webmail.mozdev.org/webmail.html, Configuring components http://webmail.mozdev.org/components.html