Icons are not always loaded

[GentlemanHal]

Bug report

Nevergreen version?
v6.0.0+2534.ce9e1c8 Quick Silver

What are you running on?
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.80 Safari/537.36

Expected behaviour?
Icons are loaded when the page is refreshed.

Actual behaviour?
Icons are only sometimes loaded when the page is refreshed.

Steps to reproduce?

1. Refresh the page a bunch

I believe this is to do with a change introduced in Chrome 83

In Chrome 83, new font loading improvements have been made to completely eliminate layout shifting and flash of invisible text (FOIT) when optional fonts are preloaded.
https://web.dev/preload-optional-fonts/

So setting the icon font to font-display: swap; (or block) will fix this.