Earlier this year, Firefox got in-content settings, doing away with the old dialog-based options menu. You may have noticed though that the old preferences menu is still there and there are no signs that the in-content settings feature is landing any time soon.
Unfortunately, that seems to be the fate of plenty of advanced Firefox features. Then again, the same is probably true for any piece of software.
Putting together a prototype that looks the part and works as intended isn't exactly trivial, but getting that prototype to a level of polish, performance and stability to satisfy the requirements of a software that is used daily by hundreds of millions of people is harder.
Which is why the new tab page took a year to become reality and the version that finally went into Firefox is spartan at best. The same is true for the in-content preferences feature.
While most of the work was done in spring, as part of a student project at Mozilla, the feature still isn't being considered complete and, as such, it's not enabled in Firefox. It's built into the browser
though, so if you want to try it out, all you have to do is mess around in about:config. All you have to do is set "browser.preferences.inContent" to "true" and restart the browser.
The feature is mostly complete and functional, but you can disable it the same way you enabled it, this time by setting the config variable to "false," if you encounter any problems.
As for when we're likely to see the feature enabled by default, it's not going to be long now actually, it's on its way
to becoming the default in Firefox 18 nightly. But even if it makes it in this cycle, it's going to land at the end of the year, at best.