[Bug 1953685] [NEW] [Regression] Firefox 95 is laggy on GMA X4500 (EGL related?)

[Kevin Keijzer]

Public bug reported:

I have a couple of ThinkPads with old Intel GMA X4500 graphics. Since
the update to Firefox 95, it's been verry laggy; especially noticeable
while scrolling, for instance on about:support.

Strangely, the bug is less present when the browser window is smaller. A
maximized window lags very badly, but when resizing the window to under
~75% of the screen, it becomes a lot better.

I have tried completely clean profiles by renaming ~/snap/firefox, but
this made no difference.

I have a feeling that EGL has something to do with it. When I force the
Firefox snap to run on Xwayland by running `WAYLAND_DISABLE=1 snap run
firefox`, the bug is *not* present.

When I run the Firefox 95 deb (from jammy) on Wayland, by running
`MOZ_ENABLE_WAYLAND=1 firefox`, the bug is also present. (When I run the
deb without that environment variable, it falls back to Xwayland, which
works normally.)

However, when I set gfx.x11-egl.force-enabled to true in about:config,
it also lags.

One may assume that EGL is simply broken on this chipset, but that is
not the case. When I run the Firefox *94* deb (from focal) with
`MOZ_ENABLE_WAYLAND=1`, it works fine. Also, when I run `snap revert
firefox` to go back to 94.0.2-2, the bug is not present, even when
running on Wayland, using EGL.

I have tested the snap both on focal and jammy on the same hardware. The
bug is identical in both cases.


tl;dr: On old Intel graphics Firefox 94 was working well with GLX and EGL, while EGL seems broken on Firefox 95.

** Affects: firefox (Ubuntu)
     Importance: Undecided
         Status: New

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Title:
  [Regression] Firefox 95 is laggy on GMA X4500 (EGL related?)

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