I have a gmail account and would like to be able to open it in my web browser directly from MailWasher. This feature can probably be implemented via some kind of special filter, or just with an user configurable toolbar item.

Also, when a mail is read, it is still present in MailWasher. Deleting it does not remove it in the inbox, but removes the entry in MailWasher. And the mails I write and send myself are also listed in MailWasher (although they are automatically marked for deletion.)
I'm not sure it is possible to improve that a bit (so that the gmail inbox behaves like a normal mailbox), or if it's a limitation of the gmail POP server.

Hope you understand. My English is not very good.

Go to options and use the specify option for launch e-mail program to launch your browser with your g-mail program loaded.