Doesn't work anymore. Aditionally, "The last release was last year, but the version that gets installed from browsers doesn't match the GitHub releases. I'm running v3.0.0.9, the latest GitHub release is v3.0.0.3, and is the only release. Some work is still being done, but as pointed out various PRs appear to be in limbo. Clarification on the state of the project would be good to know."

edit: thanks for adding the v3.0.0.10 on GitHub. But the firefox.zip does not contain a valid manifest, so it can't be installed.