Before we get to the marquee features, though, there are two changes which you may not immediately notice. The first is a mitigation for a long-standing issue where some malicious sites keep popping up authentication modals using HTTP Auth. Essentially you can't do anything with the window until the modal is dealt with, so the site just asks for your credentials over and over, ultimately making the browser useless (as a means to make you call their "support line" where they can then social engineer their way into your computer). The ultimate solution is to make such things tab-modal rather than window-modal, but that's involved and sort of out of scope, so we now implement a similar change to what current Firefox does where there is a cap of three Cancels. If you cancel three times, the malicious site is not allowed to issue any more requests until you reload it. No actual data is leaked, assuming you don't type anything in, but it can be a nasty denial of service and it would have succeeded in ruining your day on TenFourFox just as easily as any other Firefox derivative. That said, just avoid iffy sites, yes?
The second change is more fundamental. For Firefox 66 Mozilla briefly experimented with setting a frame rate cap on low-end devices. Surprise, surprise: all of our systems are low-end devices! In FPR13 and prior, TenFourFox would try to push as many frames to the compositor as possible, no matter what it was trying to do, to achieve a 60fps target or better. However, probably none of our computers with the possible exception of high-end G5s were probably achieving 60fps consistently on most modern websites, and the browser would flail trying to desperately keep up. Instead, by setting a cap and enforcing it with software v-sync, frames aren't pushed as often and the browser can do more layout and rendering work per frame. Mozilla selected a 30fps cap, so that's what I selected as an arbitrary first cut. Some sites are less smooth, but many sites now render faster to first paint, particularly pages that do a lot of DOM transforms because now the resulting visual changes are batched. This might seem like an obvious change to make but the numbers had never been proven until then.
Mozilla ultimately abandoned this change in lieu of a more flexible approach with the idle queue, but our older codebase doesn't support that, and we don't have the other issues they encountered anyway because we don't support Electrolysis or APZ. There are two things to look at: we shouldn't have the same scrolling issues because we scroll synchronously, but do report any regressions in default scrolling or obvious changes in scroll rate (include what you're doing the scrolling with, such as the scroll bar, a mouse scroll wheel or your laptop trackpad). The second thing to look at is whether the 30fps frame rate is the right setting for all systems. In particular, should 7400 or G3 be even lower, maybe 15fps? You can change this with layout.frame_rate to some other target frame rate value and restarting the browser. What setting seems to do best on your machine? Include RAM, OS and CPU speed. One other possibility is to look at reducing the target frame rate dynamically based on battery state, but this requires additional plumbing we don't support just yet.
So now the main event: H.264 video support. Olga gets the credit here for the original code, which successfully loads our separately-distributed build of ffmpeg so that we don't run afoul of any licenses including it with the core browser. My first cut of this had issues where the browser ran out of memory on sites that ran lots of H.264 video as images (and believe me, this is not at all uncommon these days), but I got our build of ffmpeg trimmed down enough that it can now load the Vimeo front page and other sites generally without issue. Download the TenFourFox MP4 Enabler for either G4/7450 or G5 (this is a bit of a misnomer since we also use ffmpeg for faster MP3 and VP3 decoding, but I didn't want it confused with Olga's preexisting FFmpeg Enabler), download FPR14b1, run the Enabler to install the libraries and then start FPR14b1. H.264 video should now "just work." However, do note there may be a few pieces left to add for compatibility (for example, Twitter videos used to work and then something changed and now it doesn't and I don't know why, but Imgur, YouTube and Vimeo seem to work fine).
There are some things to keep in mind. While ffmpeg has very good AltiVec support, H.264 video tends to be more ubiquitous and run at higher bitrates, which cancel out the gains; I wouldn't expect dramatic performance improvements relative to WebM and while you may see them in FPR14 relative to FPR13 remember that we now have a frame rate cap which probably makes the decoder more efficient. As mentioned before, I only support G4/7450 (and of those, 1.25GHz and up) and G5 systems; a G4/7400 will have trouble keeping up even with low bitrates and there's probably no hope for G3 systems at all. The libraries provided are very stripped down both for performance and to reduce size and memory pressure, so they're not intended as a general purpose ffmpeg build (in particular, there are no encoders, multiplexers or protocols, some codecs have been removed, and VP8/VP9 are explicitly disabled since our in-tree hyped-up libvpx is faster). You can build your own libraries and put them into the installation location if you want additional features (see the wiki instructions for supported versions and the build process), and you may want to do this for WebM in particular if you want better quality since our build has the loop filter and other postprocessing cut down for speed, but I won't offer support for custom libraries and you'd be on your own if you hit a bug. Finally, the lockout code I wrote when I was running into memory pressure issues is still there and will still cancel all decoding H.264 instances if any one of them fails to get memory for a frame, hopefully averting a crash. This shouldn't happen much now with the slimmer libraries but I still recommend as much RAM as your system can hold (at least 2GB). Oh, and one other thing: foxboxes work fine with H.264!
Now, enjoy some of the Vimeo videos you previously could only watch with the old PopOutPlayer, back when it actually still worked. Here are four of my favourites: Vicious Cycle (PG-13 for pixelated robot blood), Fired On Mars (PG-13 for an F-bomb), Other Half (PG-13 for an F-bomb and oddly transected humans), and Unsatisfying (unsatisfying). I've picked these not only because they're entertaining but also because they run the gamut from hand-drawn animation to CGI to live action and give you an idea of how your system performs. However, I strongly recommend you click the gear icon and select 360p before you start playback, even on a 2005 G5; HD performance is still best summarized as "LOL."
At least one of you asked how to turn it off. Fortunately, if you never install the libraries, it'll never be "on" (the browser will work as before). If you do install them, and decide you prefer the browser without it, you can either delete the libraries from ~/Library/TenFourFox-FFmpeg (stop the browser first just in case it has them loaded) or set media.ffmpeg.enabled to false.