Ok, I got them to show on all browsers, except the Firefox Developer Edition. Developer Edition has always higher version number than ordinary Firefox, so the icons might be missing again soon enough.

After updating Woodmart theme I did the following:
In Settings-> Performance -> Fonts & Icons tab I selected:
“font-display” for icon fonts: Swap
“font-display” for icon fonts: Swap
Font Awesome library: Don’t use
Preload key request for “woodmart-font.woff2”: OFF
Save options <- click

Then in Settings-> Other
Slider Revolution inherit theme font: ON
Save options <- click

I had pretty much the same settings before, but I set the settings to something else temporarily, saved and then set the configuration above.

After that I got the icons back again (for now) but I have no confidence that it lasts. I will continue the topic when they go missing again. Just to help anyone else who’s icons go missing every now and then.