Here are some examples. I have a lot more. It's hard to develop a new browser when everyone has a "must have" site that serves degraded or broken markup to everything except WebKit.

Google Maps in WebKit: http://dl.dropbox.com/u/2128410/maps-webkit.png

Google Maps in Firefox: http://dl.dropbox.com/u/2128410/maps-firefox.png

Gmail in WebKit: http://dl.dropbox.com/u/2128410/gmail-webkit.png

Gmail in Firefox spoofing a WebKit UA header: http://dl.dropbox.com/u/2128410/gmail-firefox-webkit-ua.png

Gmail in Opera or Firefox normally: http://dl.dropbox.com/u/2128410/gmail-opera.png

Twitter in WebKit: http://dl.dropbox.com/u/2128410/twitter-webkit.png

Twitter in Firefox: http://dl.dropbox.com/u/2128410/twitter-firefox.png

These are just some of the big-name sites that I knew off-hand. At least these high-budget sites usually have some sort of fallback. Some sites just have degraded styles; some have parts that are unreadable. Some provide partial functionality to non-WebKit browsers; others won't work at all (like Bing web search in Opera).

This is becoming more common on the desktop too, with sites like Tweetdeck that just lock out non-WebKit browsers using UA sniffing: https://web.tweetdeck.com/web/unsupported.html

Great examples. I think even the WebKit touch events version of Google Maps is broken, but, not being a Gmail user, I had no idea they were blowing it that badly. I use both Maps and Twitter every single day, but -- and here's another facet of the problem -- always through applications. I encounter Maps embedded in (WebKit) pages only enough to be annoyed by them, and I haven't been to twitter.com in at least a year. This is eye-opening. Clearly we're not just talking about drop shadows.

Developers are ambitious. We want to build better apps now not in 6 years time when the W3C have finally moved on this or that issue. Right now webkit is enabling developers so it wins. Supporting every browser is either economically not viable or not possible without sacrificing features. If mobile firefox wants to compete it needs to step up. If developers are voting with their feet and actually using webkit features in the wild then they are defacto standards. Firefox better pull its finger out or it will die.

In all of the above cases, Firefox actually does support the features used by these sites, and has for a long time (years, in some cases). In most cases the developers would just need to tweak their UA sniffing, or add some -moz properties alongside the -webkit properties in their styles.

For some technical details, see for example http://bugzil.la/668218

Mozilla does need to "step up" -- but work required to fix cases like these is not technical. It's about gaining user marketshare, developer mindshare, and improving the standards process.

It's true, and since everyone upgrades their phones every two years it's not like there's a chance of older versions of a hegemonic browser haunting developers' dreams, right?