Search multiple boxes

I find searching via thunderbird very hard, so I'd love to use mutt's great searching capabilities to search in my thunderbird emails. They are organized in multiple mbox files (one per folder). The issue is that I can't find how to search via multiple mbox files. If I do:

$ mutt -R -f -f ImapMail/imap.googlemail-1.com/INBOX -f ImapMail/imap.googlemail-2.com/INBOX

only the second mailbox is searched. Ideally, I'd even like Mutt to automatically find all mbox files in a given directory/thunderbird profile, as a new mbox file will be added each time I create a new folder (and I might have many in a near future). In thunderbird at least, it seems that one can identify mbox files as they are usually placed next to a file with the same name with the extension .msf, like:

$ ls ImapMail/mail.my.imap.fr/Myfolderlevel1.sbd/Myfolderlever2.sbd/
Foo  Foo.msf  Bar  Bar.msf  Baz  Baz.msf

Ideally, mutt would also allow me to filter email based on their folder name.