are there any plans for Readium as extension for Firefox and Safari?
No, not at this time. In fact, the Chrome extension is probably going to have to change significantly in 2017 as Google has said that they will be deprecating the "Chrome applications", which is what our Chrome Extension is since the introduction of Packaging 2.0 eaarly in 2016. We are currently looking to replace the Chrome Extension with some form of hybrid application like Electron/Node.js or similar. Biggest challenge will be migrating all the users' libraries.
Hope this answers your question.
Many thanks for the prompt reply. Good luck!