Just to update everyone.
Chrome vs FireFox:
I dislike Chrome's visual appearence, and the idea of Google's spying. For all of Chrome's supposed speed, I have never seen it run better than FF, and in some cases Chrome chugged under the load of tabs/gifs/videos I have open. I tend to go down a forum page, middle-mouse-click any threads that look interesting, end up with like 60 tabs open, read them, relaod them, and then read what's new since I opened the tab. I am also pretty sure that whilst Chrome has addons, it wouldn't have the right addons to do everything I do in FF between addons, userscripts and stylish tweaks. I mean just look at this lot up-top, and that's without looking into my hamburger menu:
Win8 vs Win10; Flash
Now, with regard to Windows 8.1 vs Windows 10. This is the same FF profile on both machines, copied onto a USB drive, replacing the fresh install of FF's default profile folder contents etc. So everything is set-up as I like it. Thus, I know I have the exact same version of FF and the exact same versions of the various addons in play on both machines. The only thing that has changed is the operating system. Flash works on Win8.1, and not on Win10. Yes I know Flash is depreciated, but just because YouTube uses HTML5 doesn't mean that countless other video sites on the web also use HTML5 yet.
Everything else on Win10 works, except Flash. In order to get Flash working I have to go from normal FF release channel, version 45, all the way to Nightly release channel, version 48. God knows when that will make it's way down to the normal FF channel, could be like 6 months from now. Of course using this early version of Nightly breaks other addons which I consider to be dealbreakers. So whilst Flash works, I need the addons as they are integral to the way I use FF. I currently find myself jumping back and forth between FF and Nightly, depending on if I need my addons for general browsing, or I need Flash. I suppose I will check out a FF fork like PaleMoon though, as a possible temporary solution for this Win10 machine, and wait for that Nightly build to release as regular FF, with the hope that addon developers will support the then-current release, whereas at the moment Nightly compatability is asking a little too much.
I have no idea why Flash is busted on current FF for me, specifically on Win10. But like I said, it happens in FF safe mode, on a new profile, after a win10 reset this pc, even on a fresh Windows 10 user profile (local admin verses my current microsoft account admin) etc... so it's not something I've done. It appears to be down to the combination of FF and Win10.