With the glacial pace that new technologies are becoming available in web browsers, webmasters don't bother with new technology
Might be a valid argument, if FF was only browser out there or had a market share like IE did years ago so other browsers did not matter. But neither is the case.
There are two simple facts:
Web dev's generally will not implement some new tech until majority of browsers in use support it, because otherwise they will have to do two or more versions of everything they do (and we got tired of that YEARS ago)
Web dev's who do it for an actual living don't want to to spend all their time learning every single damn new tech that comes out, not only will their boss object to them having so much downtime but they have lives as well.
Anything new is going to spend a lot of time with amateurs, hobbiest's, 'geeks re-inventing the wheel' just to prove it can be done, who are quite happy to have something only working on one browser, before it will have wide scale uptake. So sure implement it, but there is no damn rush to get it out the door because majority of people will not be using those features for a long while
Just because the situation made it easy for Mozilla to push the web into the future once, it needs realize it cannot do it again and if it keeps trying the only likely future for the web will be one without Firefox in it