The major change in this release is additional tweaking to the MediaSource implementation and I'm now more comfortable with its functioning on G4 systems through a combination of some additional later patches I backported and adjusting our own hacks to not only aggressively report the dropped frames but also force rebuffering if needed. The G4 systems now no longer seize and freeze (and, occasionally, fatally assert) on some streams, and the audio never becomes unsynchronized, though there is some stuttering if the system is too overworked trying to keep the video and audio together. That said, I'm going to keep MediaSource off for 45.4 so that there will be as little unnecessary support churn as possible while you test it (if you haven't already done so, turn media.mediasource.enabled to true in about:config; do not touch the other options). In 45.5, assuming there are no fatal problems with it (I don't consider performance a fatal flaw, just an important one), it will be the default, and it will be surfaced as an option in the upcoming TenFourFox-specific preference pane.
However, to make the most of MediaSource we're going to need AltiVec support for VP9 (we only have it for VP3 and VP8). While upper-spec G5 systems can just power through the decoding process (though this might make hi-def video finally reasonable on the last generation machines), even high-spec G4 systems have impaired decoding due to CPU and bus bandwidth limitations and the low-end G4 systems are nearly hopeless at all but the very lowest bitrates. Officially I still have a minimum 1.25GHz recommendation but I'm painfully aware that even those G4s barely squeak by. We're the only ones who ever used libvpx's moldy VMX code for VP8 and kept it alive, and they don't have anything at all for VP9 (just x86 and ARM, though interestingly it looks like MIPS support is in progress). Fortunately, the code was clearly written to make it easier for porters to hand-vectorize and I can do this work incrementally instead of having to convert all the codec pieces simultaneously.
Interestingly, even though our code now faithfully and fully reports every single dropped frame, YouTube doesn't seem to do anything with this information right now (if you right-click and select "Stats for nerds" you'll see our count dutifully increase as frames are skipped). It does downshift for network congestion, so I'm trying to think of a way to fool it and make dropped frames look like a network throughput problem instead. Doing so would technically be a violation of the spec but you can't shame that which has no shame and I have no shame. Our machines get no love from Google anyway so I'm perfectly okay with abusing their software.
I have the conversion to platform codec of our minimp3 decoder written as a first draft, but I haven't yet set that up or tested it, so this version still uses the old codec wrapper and still has the track-shifting problem with Amazon Music. That is probably the highest priority for 45.5 since it is an obvious regression from 38. On the security side, this release also disables RTCPeerConnection to eliminate the WebRTC IP "leak" (since I've basically given up on WebRTC for Power Macs). You can still reenable it from about:config as usual.
The top three priorities for the next couple versions (with links to the open Github issues) are, highest first, fixing Amazon Music, AltiVec VP9 codepaths and the "little endian typed array" portion of IonPower-NVLE to fix site compatibility with little-endian generated asm.js. Much of this work will proceed in parallel and the idea is to have a beta 45.5 for you to test them in a couple weeks. Other high priority items on my list to backport include
allowing WebKit/Blink to eat the web supporting certain WebKit-prefixed properties to make us act the same as regular Firefox, support for ChaCha20+Poly1305, WebP images, expanded WebCrypto support, the "NV" portion of IonPower-NVLE and certain other smaller-scope HTML/CSS features. I'll be opening tracking issues for these as they enter my worklist, but I have not yet determined how I will version the browser to reflect these backported new features. For now we'll continue with 45.x.y while we remain on 45ESR and see where we end up.
As we look into the future, though, it's always instructive to compare it with the past. With the anticipation that even Google Code's Archive will be flushed down the Mountain View memory hole (the wiki looks like it's already gone, but you can get most of our old wikidocs from Github), I've archived 4.0b7, 4.0.3, 8.0, 10.0.11, 17.0.11 and Intel 17.0.2 on SourceForge along with their corresponding source changesets. These Google Code-only versions were selected as they were either terminal (quasi-)ESR releases or have historical or technical relevance (4.0b7 was our first beta release of TenFourFox ever "way back" in 2010, 8.0 was the last release that was pure tracejit which some people prefer, and of course Intel 17.0.2 was our one and so far only release on Intel Macs). There is no documentation or release notes; they're just there for your archival entertainment and foolhardiness. Remember that old versions run an excellent chance of corrupting your profile, so start them up with one you can throw away.
Finally, a good reason to dump Dropbox (besides the jerking around they give those of you trying to keep the PowerPC client working) is their rather shameful secret unauthorized abuse of your Mac's accessibility framework by forging an entry in the privacy database. (Such permissions allow it to control other applications on your Mac as if it were you at the user interface. The security implications of that should be immediately obvious, but if they're not, see this discussion.) The fact this is possible at all is a bug Apple absolutely must fix and apparently has in macOS Sierra, but exploiting it in this fashion is absolutely appalling behaviour on Dropbox's part because it won't even let you turn it off. To their credit they're taking their lumps on Hacker News and TechCrunch, but accepting their explanation of "only asking for privileges we use" requires a level of trust that frankly they don't seem worthy of and saying they never store your administrator password is a bit disingenuous when they use administrative access to create a whole folder of setuid binaries -- they don't need your password at that point to control the system. Moreover, what if there were an exploitable security bug in their software?
Mind you, I don't have a problem with apps requesting that access if I understand why and the request isn't obfuscated. As a comparison, GOG.com has a number of classic DOS games I love that were ported for DOSBox and work well on my MacBook Air. These require that same accessibility access for proper control of input methods. Though I hope they come up with a different workaround eventually, the GOG installer does explain why and does use the proper APIs for requesting that privilege, and you can either refuse on the spot or disable it later if you decide you're not comfortable with it. That's how it's supposed to work, but that's not what Dropbox did, and they intentionally hid it and the other inappropriate system-level things they were sneaking through. Whether out of a genuine concern for user experience or just trying to get around what they felt was an unnecessary security precaution, it's not acceptable and it's potentially exploitable, and they need to answer for that.
Watch for 45.4 going final in a couple days, and hopefully a 45.5 beta in a couple weeks.