So I have been trying to delve deeper into the "stuttering scrolling" issue, and I noticed some peculiar aspects on the two machines I've tested (the Tualatin and the Athlon 64 X2 at work). The issue seems to mostly affect Firefox 45 ESR SSE: if you scroll a page, in the beginning it's fine, but then it starts to stutter, even badly in some sites. Ironically enough, the site that suffers the most of this issue is Google, regardless of the user agent. But every site is affected by this, in a way or another. I have checked the browser even at stock configuration and without hardware acceleration enabled, and does the same thing, so it's a browser issue, or at least a problem caused by the way the browser renders the page.
On the other hand, the issue is mitigated drastically in New Moon 27, to the point it's almost unnoticeable, and is inexistent in K-Meleon 76 Goanna.
Now, I could have used New Moon 27 as a main browser, but unfortunately I can't since it doesn't support the earlier versions of YT 2 Player, an add-on that automatically parses Youtube links to VLC, only the latest version, which is much more bloated than the earlier ones, works.
I spent all the night sleepless in order to figure out a way to mitigate this issue, but I haven't found anything that could fix it. There might be something that could help reduce the stuttering a little bit, but I need to test it thoroughly before I can implement it in the UOC Patch. As I'm not a programmer and I can't in any way contribute to improve the source code of FF45 ESR SSE due to my lack of programming skills, I'm doing my best. Will keep you all updated.
EDIT: Nothing. None of the possible solutions I tried works. Looks like I'll have to abandon Firefox 45 ESR SSE and switch to New Moon 27, but only if I find a way to make the earliest version of YT 2 Player work on it.
The issue lies in the rendering engine of the browser, so no patch can fix that, unless the code of the browser itself is modified.