This just started happening today. The whole row of icons to the right of my username on the third row of the upper right corner are invisible unless and until I hover the pointer over them (even then only the one I am currently pointing to is visible).
I'm browsing on a Windows desktop using Firefox 84.0.1 (64-bit), with the following plugins
Ublock Origin (but it's turned off for Penny Arcade and the forums)
NoScript (but I allow everything on this site except google analytics)
Malwarebytes Browser Guard (again, allowing everything but google analytics)
It's not a big deal, but I figured I should pass it along.