45.7 will not have substantial changes and I don't anticipate doing a beta. However, one change I do intend to make is to mirror Mozilla's work on updating default settings, starting with layout paint delay. The rationale for delaying layout painting specifically was to wait for sufficient data to come through rather than guessing an incorrect layout with incomplete data that then has to be invalidated: without the delay, although the screen would be busier, the browser often would end up taking more total wall-clock time on wasted work. Now that data arrives faster on most people's systems today than in the days of dialup and low-speed DSL, it's time for these older settings from another age to be re-examined, and paint delay is probably the most visible one of those settings.
Stuff like that has long been part of the various unofficial Firefox "optimization guides" that circulate, including Erik's set for TenFourFox. I have generally avoided comment on his recommendations (except for a couple that I knew would be net negative for most users) because as far as I'm concerned, it's your computer and you can tune it as you like -- just don't file bug reports if you muck it up because some of those settings have undesirable side effects in edge cases. For that reason I have declined to move too far from the Firefox base settings because the browser out-of-the-box has to work for as many systems as possible in as many situations as possible, and one thing unique to us is we still do have a substantial minority of users using Power Macs on dialup networks. One user sticks out in my mind who is a missionary in the mountains north of Myanmar and completely reliant on the modem in his G4 mini. We don't want to unnecessarily tank these users with settings that are overly optimistic about bandwidth availability, so whatever setting Mozilla determines for Firefox users at large may not be the best fit for our legacy population.
In bug 1283302, Mozilla settled on 5ms for desktop users and left Firefox Android at 250ms (which is actually smaller than a refresh tick, so near as I can determine it might as well be zero). Since we're not in the same processing class as current machines by a long shot and we do need to still support users with limited bandwidth, I think a safer setting will be 100ms, which as an otherwise arbitrary number seemed not to regress anything on the local machines. If you want to try this, go into about:config, create a numeric pref nglayout.initialpaint.delay if it does not already exist, and enter a value of 100. Optimally it might be nice to have such settings specific to each architecture build and tuned accordingly, but that's something to consider at a later time. If you have other reasonable recommendations for this setting, do post them in the comments, along with the specifications of the system and network you tried it on. I will consider other changes in future versions as Mozilla re-examines them internally.
Meanwhile, I'll be on a plane to Australia next week on what may be my last Spaceseat flight on Air New Zealand, which I loved. Before I do, however, I'll be stopping by the parents' house to look at the dual 2.5GHz G5 they use for uploading their church videos. My suspicion is the liquid cooling system blew. Currently I am sad.