browser-extensions | Browser extensions for Terms of Service ; Did n't Read | Browser Plugin library

I was evaluating the development of a cross browser extension (supported browsers IE, Chrome, Safari and Firefox): I was looking for a cross-browser development framework, in order to avoid the code duplication and to fasten the building process.
I had a look at this question, and I visited the suggested websites: every company seems to have stopped working on the product.
I read this article too, whose conclusion seems to dissuade developer from the building of a cross browser extension.
So, I have a few questions:
- Does anyone of you recommend a cross browser extension development framework, which is suitable for the purpose I described above?
- Is there a reason why the above frameworks are not maintained anymore?
- Does anyone discovered a more efficient way to pursue the same aim without developing a browser extension?

Thanks so much,
Daniele