Browsing habit seems to be one of the big factors on anonymity, and i am wondering if there is possibility that technology wise Tor can be written as a Plugin on top of Mainstream browsers, so the adoption rate would increase, hence a more effective/faster Tor network, and people would use it as a habit usage instead of targeted use.
The Torbutton - which is a major part of the Tor Browser - is effectively a Firefox extension. However, it can only be used as part of the Tor Browser itself, rather than be added to a vanilla Firefox instance. (I think it used to be a standalone add-on that could be added to a vanilla Firefox instance - someone with longer experience can correct me here.) The Torbutton code can be found in its GitHub repository.
I believe the Tor Browser was developed to solve problems that couldn't be solved using an extension/add-on alone. Have a look at the Tor Browser Design and Implementation document, specifically section 2. Design Requirements, which enumerates, surprisingly, what was required of the browser.
With regards to Chrome, have a look the previous threads which explain why this wouldn't be possible. (i.e. The API problem mentioned by Alexey.)
Impossible. And not due to the API limitations. The reason is that browser plugins can be not just disabled, but controlled from browser, enlisted e.t.c... All-in-one conception is turning dead when there's too much things aboard : every piece of software must do it's job an it's job only. Another argument is that the browser is not an ultimate container for it's plugins : the spectacular example is Flash or Silverlight - the will ignore your browser's connection settings and no plugin or proxy will save you from a data/location/identity leak.
Although the idea may seem intriguing, the problem of use the Tor network to browse the internet is much deeper than that. The Tor browser is a separate browser because when you access the Tor network it changes how you get data from servers.
With mainstream browsers (IE, Chrome, Firefox, ...) the developers of them would have to implement the Tor protocol in the core engine to enable Tor browsing. This is because instead of using regular DNS and directly connecting to the web server, the Tor browser will first encrypt your data and proxy your request through at least 3 other relays.
Only at the point when the request comes out of the (3)rd exit relay does the request use the normal Internet and access the server.
Also because there is the barrier of knowing about Tor and the TBB, it makes it so that the regular oblivious internet can't be sent a spam email to a child porn website and be guaranteed the link will open.
Hope this helps!