I am noticing graphical glitches after installing the Firefox Snap, probably the confinement is blocking the access to something (shaders?) according to the debugger. It happens on surviv.io too with a mild graphical corruption and it’s not a unity games I think.
examples:
https://webassembly.org/demo/Tanks/ pink
https://snootboop.itch.io/november completely red
I tried https://snootboop.itch.io/november and https://webassembly.org/demo/Tanks/ in snapped firefox but they both reported that my browser didn’t support webgl at all, so I can’t verify this problem 
I should have specified that I am using the open source AMD drivers, I am not aware if for instance the snap can access the necessary bit of the proprietary nvidia driver for webgl. Is this your case maybe?
For me it is not a problem with webgl (every example from the threejs or babylonjs sites seems to work), but games made with unity and that webassembly demo seems to store assets that are inaccessible from the snap.
This is the error from the console
Error loading this URI: Could not load the source for blob:https://webassembly.org/884c7645-82de-405e-b57a-54a100fc19b1.
[Exception… “Failed to open input source ‘file:////webassembly.org/884c7645-82de-405e-b57a-54a100fc19b1’” nsresult: “0x80520015 (NS_ERROR_FILE_ACCESS_DENIED)” location: “JS frame :: resource://devtools/shared/base-loader.js -> resource://devtools/shared/DevToolsUtils.js :: mainThreadFetch :: line 645” data: yes]
Stack: mainThreadFetch@resource://devtools/shared/base-loader.js -> resource://devtools/shared/DevToolsUtils.js:645:5
_getSourceText/<@resource://devtools/shared/base-loader.js -> resource://devtools/server/actors/source.js:410:27
Line: 645, column: 0
I have reported the problem on the Mozilla bug tracker
It is fixed for me in 61.0.2-1
