Choosing the right profile for the job is absolutely crucial. The combinations of x264 parameters can be pretty arcane, and they sometimes change from one x264 version to another. There's a pretty active community on forum.doom9.org maintaining collections of profiles for MeGUI, some of those are excellent.
E.g., it is totally within the realm of possibility to put two hours of 1080p content on a single-layer DVD (4.4GB), in a format compatible with any Blu-Ray player out there (AVCHD, a subset of the Blu-Ray standard that accepts DVD as the storage layer), while keeping video quality at a very high level - basically indistinguishable from commercial Blu-Ray discs. But using a good encoding profile, feeding the appropriate parameters to x264, is the single most important factor in achieving that goal.
MeGUI is hardly necessary - x264 has a good set of presets and tunes built in to begin with. --preset veryslow --tune film/animation/grain will already get you very far, beyond that pretty much the two most important things to possibly tweak are the strengths of AQ and psychovisual optimizations (--aq-strength and --psy-rd).
>it is totally within the realm of possibility to put two hours of 1080p content on a single-layer DVD (4.4GB), in a format compatible with any Blu-Ray player out there (AVCHD, a subset of the Blu-Ray standard that accepts DVD as the storage layer), while keeping video quality at a very high level - basically indistinguishable from commercial Blu-Ray discs.
You might get away with an hour of almost-transparent content if it's not particularly bitrate-demanding, but two hours of live action will not look "indistinguishable from commercial Blu-ray discs". 5 Mbps High Profile L4.0 H.264 just won't look as good as ~30-40 Mbps H.264 High Profile L4.1 H.264 commonly found on BDs (unless the BD is really screwed up). At 720p you'd get pretty good results, though.
Firefox is getting MP4 support. Firefox OS and Firefox for Android on some devices has it already. Support on some versions of Windows is in nightly builds hidden behind a preference setting. Linux support is hidden behind a build switch. At some point when these backends are stable they'll be in normal builds.
Firefox getting mp4 support doesn't change anything, people are already using firefox right now, and it doesn't support mp4 right now. The fact that the marketshare of non-mp4 firefox will eventually end up small enough that most people ignore it doesn't mean mp4 is all people need right now.