Hi Mo,
Although browser extensions are great, it’s not possible for us to actively support twitter.com as a development environment. Substantial redesigns are reasonably rare (once every 12–18 months over the past few years), so for the most part you shouldn’t find yourself too regularly disrupted. However, the content of pages is constantly changing, and it is not consistent for each user due to their preferences, and because we constantly run experiments of new features large and small before they are announced (some of these, of course, never launch.)
There’s no beta site with all these features enabled (that too is impossible as many experiments are variations of the same change), and it’s not viable or efficient for feature teams to coordinate announcing subtle mark-up changes like this.
With any kind of user-scripting on the web I can only offer the regular advice: Code defensively against change, and make full use of the canvases that the browser itself provides for your UI.
Good luck,
Ben