Any existing Firebreath plugin uses NPAPI, and will thus be blocked by default. This isn't the same as not working at all, which will come later; see my answer to this question.
The links in the comments above are probably the best resource for discussions of future options for a cross-browser framework to do what you want. However, before you can reasonably figure out what the best replacement technology for your case is, you'd need to know more about what the plugin does, because there's no one-size-fits-all replacement for all of NPAPI (by design).