Right now, for example, Chrome has taken so many shortcuts to try to improve responsiveness that it's just plain broken when it comes to refreshing the page layout or even just repainting in timely fashion after a DOM or CSS change. That means even popular UI toolkits have trouble with basic things like displaying dialog-style content, an everyday task if you design browser-based UIs. And because it's not repainting properly in the first place, presumably because some internal events aren't invalidating some cached data they should be, there's no clear workaround nor any guarantee that you'll get the same breakage from one update to the next.
This one sounds like a biggie. Do you have a reproducible test case to demonstrate this behaviour? I know it's time consuming, but you really should file bug reports for this type of stuff. I've filed a number over the years and they do get worked on and they do get fixed (sometimes a lot quicker than you'd imagine).
Are you building apps or websites? I can't imagine seeing the fundamentals of layout, box model etc breaking from one release to the next.
On the other hand if you're building applications that rely on vendor prefixes then I agree you do have to tread carefully.
Moving on to Firefox, when they pushed out the LTS release a few weeks ago, they included a bug that basically stuffed every site that used such obscure technologies as Flash and Java applets.
Software breaks life goes on. If i'd never personally shipped a line of code with a bug in it I might complain louder.
The pace of innovation is blistering. Just look at all the stuff that's come out recently websockets, webgl, flex-box, css columns, dnd, offline storage, css animation, audio, video, filreader, canvas the list goes on.
If you're building just another brochure website none of this will interest you. On the other hand life is sweet if you have the luxury of using these features. Sure they're sometimes broken. Sure we're all effectively beta testers. But everyone knows that.