WebGL not working with Firefox, Intel HD graphics, ArchLinux, and Thinkpad T540p

Is anyone having issues getting webGL to work on Firefox? https://get.webgl.org/ It was working 1-2 months ago, but is now broken. Works fine on Chromium. I set webgl.force-enabled; to true, but no luck. Not sure if there are other promising configs - I did a search for "webgl" in the about:config page, but I can't find anything promising. Here is some info on my system:

[my-pc]/home/me$ firefox --version
Mozilla Firefox 43.0.4
[my-pc]/home/me$ uname -a
Linux my-pc 4.3.3-3-ARCH #1 SMP PREEMPT Wed Jan 20 08:12:23 CET 2016 x86_64 GNU/Linux
[my-pc]/home/me$ lspci | grep VGA
00:02.0 VGA compatible controller: Intel Corporation 4th Gen Core Processor Integrated Graphics Controller (rev 06)

I also tried starting firefox with --safe-mode and with --profile /path/to/empty/folder but webGL is still not working. Any suggestions?