Location has been granted. Nothing to do with themes or add-ons since the troubleshoot mode makes no difference.

I have now noticed several more icons missing, all are at the leftmost column of the page. See the attached pic.

Yes, Private or Troubleshoot mode, neither makes any difference as far as missing icons are concerned. Whether I signed in or not makes no difference. There are 3 add-ons: SingleFile, uBlock, Dictionary Anywhere.

Chrome has all three and more, yet it has no issues. In Windows, are you seeing all those beautiful icons?

It appears to work for me, please verify. see screenshot Are you in X11 or Wayland? Try downloading Firefox from Mozilla, run it from the folder and see if you have the same issue. https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

Operating System: openSUSE Tumbleweed 20220823 KDE Plasma Version: 5.25.4 KDE Frameworks Version: 5.97.0 Qt Version: 5.15.5 Kernel Version: 5.19.2-1-default (64-bit) Graphics Platform: X11 Processors: 4 × Intel® Core™ i7-4810MQ CPU @ 2.80GHz Memory: 31.0 GiB of RAM Graphics Processor: Mesa Intel® HD Graphics 4600 Manufacturer: Hewlett-Packard Product Name: HP ZBook 17 G2

That is the Material Icons font.

These days a lot of websites use an iconic font to display small images. If you disable website font via "Allow pages to choose their own fonts, instead of my selections above" then such fonts are disabled and you may see little boxes with the hex code of the character or text labels instead.

This can happen if you block remote fonts that are downloaded from internet or otherwise block website fonts.

Make sure you allow pages to choose their own fonts.

• Settings -> General -> Language and Appearance -> Fonts -> Advanced
  [X] "Allow pages to choose their own fonts, instead of your selections above"

If you use uBlock Origin make sure not to "block remote fonts".