about:logo still contains Firefox logos

We fixed a lot of Firefox logos by replacing them with their Tor Browser counterparts in #25702 (moved). However, about:logo still shows a combination of Firefox icon and Firefox related text. We'd want to have a respective Tor Browser logo/icon for that one as well. (Discovered while trying to figure out what the purpose of the unmodified icons in the mobile folder is; mobile is affected, too).

The icon in question is located in browser/branding/$series/content/about.png for desktop and mobile/android/branding/$series/content/about.png for mobile.