The Chrome extension example is pretty weak. I don't know why anyone would think you could get something form the Chrome Web Store using Firefox.
It was a huge effort to leave the Microsoft lockin back then, and the Web(togheter with Linux, GNU and open source), a open platform did it! It was like the 'man in the moon' back then; And now we are simply going back, and we are just repeating the same mistakes again
1. The vast majority of things on the Chrome Web Store are web apps, not browser extensions.
2. Should GMail really only be able to work offline in Chrome?
As for your second point, no it shouldn't :( Seems they built this "offline app" stuff into Chrome and plan on keeping it there.
Does anyone but me remember offline email clients (hello Thunderbird!)? You can take Gmail offline already.