Loqui IM (included in Firefox Marketplace) is a chat client supporting XMPP.
It’s FLOSS, licensed under GNU AGPLv3.
You’ll notice that it’s an alpha version. Currently, there are some bugs with the XMPP implementation, but it seems to be actively developed. I can’t recommend it as a stable solution at the moment, but I’m sure it will improve over the next time.
A possible showstopper for many: Loqui IM’s roster doesn’t show who is online! You have to open a chat with a contact to see if they are online. It seems to be intentional that way.
Issues can be reported on GitHub. (There is also a mailing list for developers, but it seems to be in Spanish only.)
About the optional features:
- multiple accounts: yes, supported
- end-to-end encryption: yes, it supports OTR
- MUCs: yes, supported