Enable WebRender layer compositor

[alcortazzo]

This change adds the gfx.webrender.layer-compositor preference to user.js and enables Firefox’s newer layer compositor.

References:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1945683
• https://www.reddit.com/r/firefox/comments/1p58qre/firefox_is_getting_ready_to_make_youtube_fast
• https://www.ghacks.net/2025/11/24/these-two-tweaks-should-improve-firefoxs-performance-on-youtube-significantly

[yokoffing]

I also saw this Reddit post lol

[alcortazzo]

While I do feel this parameter improves performance (I tested it on two MacBooks (M1 Pro and M3 Pro), and Windows PC (4060ti and ryzen 9 9950X), and many heavy pages feel faster), I didn’t see any noticeable difference in the Speedometer 3.1 benchmark. Maybe I picked the wrong benchmark for this kind of change, or maybe it’s just a placebo (I don’t think so). That said, I’d be happy if we as a community (or you as the maintainer) could at least consider adding this parameter to default config file.

[yokoffing]

I didn’t see any noticeable difference in the Speedometer 3.1 benchmark.

If you look at the bug report (first link), one of the first things I noticed was the Speedometer regression. I haven’t read enough to know if they fixed the regression, nor have I tested on my own machines.

I did try on my potato work computer and didn’t notice much of a difference. But I need to intentionally test a few sites.

The pref switch is for Windows. I haven’t looked to see the values for Linux and macOS.

[yokoffing]

We'll add in #447 and monitor for regressions.