I'll open with my standard disclaimer: I haven't investigated this in any detail, nor do I have any experience developing browser extensions. That said, this is a blog - rampant, uneducated speculation is my speciality.
Mozilla is releasing a new extensions framework named "Jetpack". The main feature of this API is that installing an extension would not require a browser restart. Now, its still in early beta, and the security framework hasn't been completed yet, but, if I may speculate, this may be a useful model for browser extensions. The main thing that catches my attention is that this appears to be an API of some sort - if I understand the current extensions model correctly, they basically work as patches to the browser, re-writing chunks of its code. (The last sentence implies that Jetpack will deprecate the current extensions model - as well as I can tell, that is not the case.) Now, back to this being an API - instant install/uninstall (I assume) is nice - a good security model is a must have - but, beyond that, I could see other browsers implementing Jetpack. Firefox is great, and I love its customization options, fair desktop integration, and good rendering - but it is a bit heavyweight - some of the webkit browsers coming up show promise, but the customization is a bit lacking - this could be a useful common ground. Just saying.