Can't post this on GitHub, as Microsoft has blocked my GitHub account.
Maybe it is a reply to https://www.reddit.com/r/firefox/comments/1mbamon/firefox_141_lost_minimise_maximise_and_close/
I am not talking about these icons disappearing all together, though.
This issue almost made me tear my hair out (as dumb AI was only giving fake solutions for hours, as per usual), and the author of Second System mentioned a solution.
I just had to install the Segoe Fluent Icons font, which can be downloaded directly from microsoft:
https://download.microsoft.com/download/8/f/c/8fc7cbc3-177e-4a22-af48-2a85e1c5bffb/Segoe-Fluent-Icons.zip
Voilá, the button icons are now back!
So Segoe Fluent Icons should be installed alongside r3dfox if not available in the system...
Yeah this is known issue. Not really sure what to do about it though. idk how to bundle the font, or any alternative font I could use for pre-10.
You could always simply unpack it in the installation folder / put it in the portable archive in a folder called INSTALL_THIS_FONT_IF_SYSTEM_BUTTONS_ARE_RECTANGLES (written exactly like this)
It is a strange way of solving it, a hacky one, sure. And it will work better for the portable version, as it would be plain to see while extracting and making a shortcut.
I do no use the installation version (haven't actually checked if you even have that) - maybe a custom step should be added, telling the user that if that is the case, they should go install that font? Or perhaps running a script that detects a win version less than 10, and if it finds it, it will attempt to install the font via commandline? Hmm. So many options!