Google Maps broken with Mesa-20.1 and i965 driver

Hi,

this problem might not be related to Mesa at all. Since it appeared after an
upgrade from Mesa-17.2.8 to Mesa-20.1.1 (compiling from source, same problem
with Mesa-20.1.2), this list is my best bet for the moment.

After the upgrade, Google Maps stopped working with firefox (currently
version 78.0.1). More exactly: the street map is drawn normally, even if it 
is not in cache, but after a few seconds it is replaced by a grey plane with 
some street labels drawn in an unreadable font. Zooming still works. 
No error messages at all, neither from X11 nor firefox. Other Webgl pages work 
normally. https://webglreport.com/ is happy.

This is on Linux with X11, i965 driver, libdrm-2.4.102. If I replace 
i965_dri.so with the old version from Mesa-17.2.8, the problem is gone.

Any help is greatly appreciated.

Thank you, and best regards,
Erik