Floating point framebuffer test fails on Firefox

This test fails in Firefox on Linux. Does not fail in Firefox on Windows, or in Chrome at all.

http://localhost:8080/Specs/SpecRunner.html?spec=Renderer%2FFramebuffer%20draws%20to%20a%20half%20floating-point%20color%20attachment

```
Renderer/Framebuffer draws to a half floating-point color attachment

Expected context to render 14336,15872,16384,15360, but rendered: 0,0,0,0
```

This fails in the same way in 1.67 as well: https://cesium.com/downloads/cesiumjs/releases/1.67/Specs/SpecRunner.html?spec=Renderer%2FFramebuffer%20draws%20to%20a%20half%20floating-point%20color%20attachment

Windows WebGL report:

```
Platform: Win32
Browser User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.132 Safari/537.36
Context Name: webgl
GL Version: WebGL 1.0 (OpenGL ES 2.0 Chromium)
Shading Language Version: WebGL GLSL ES 1.0 (OpenGL ES GLSL ES 1.0 Chromium)
Vendor: WebKit
Renderer: WebKit WebGL
Unmasked Vendor: Google Inc.
Unmasked Renderer: ANGLE (NVIDIA GeForce GTX 1080 Ti Direct3D11 vs_5_0 ps_5_0)
Antialiasing: Available
ANGLE: Yes, D3D9
Major Performance Caveat: No
```

Linux WebGL report:

```
Platform: Linux x86_64
Browser User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.79 Safari/537.36
Context Name: webgl
GL Version: WebGL 1.0 (OpenGL ES 2.0 Chromium)
Shading Language Version: WebGL GLSL ES 1.0 (OpenGL ES GLSL ES 1.0 Chromium)
Vendor: WebKit
Renderer: WebKit WebGL
Unmasked Vendor: Intel Open Source Technology Center
Unmasked Renderer: Mesa DRI Intel(R) HD Graphics 630 (Kaby Lake GT2)
Antialiasing: Available
ANGLE: No
Major Performance Caveat: No
```

Start by running the linked Renderer/Framebuffer spec in Firefox on Linux and compare its WebGL report with the provided Windows and Linux reports. Trace the half floating-point color attachment path and its test expectations; done means the Firefox/Linux test passes or the platform limitation is documented with a clear, reproducible diagnosis.

Written by the indexing model from the issue text.