This happens to me all the time. It’s a known issue which has been reported multiple times; #1375 appears to be the primary issue for this in the UI repo.
I’m the person who reported that. As a temporary workaround I’m using the Favicon Switcher extension (with a smaller file). You have to do it per Firefox instance; the settings aren’t synced.
My very first WM was Blackbox, back in 2000, and I imprinted on it like a baby duck, so today I still mostly use Fluxbox. It’s abandoned and unmaintained, but still works (for now). It’s very minimalist and lightweight. When it finally dies completely I guess I’ll finally learn how to use a tiling WM.
(I use Gnome on a laptop with a HiDPI screen, because that was too annoying to configure correctly on Fluxbox. It’s… fine. I added a bunch of customisations and it mostly stays out of my way, which is what I want in an environment.)
No matter what WM/DE I use, I always add a dropdown / “quakelike” terminal application – I previously used Yakuake, but switched to Guake. It uses a hotkey to show / hide a terminal (and you can use multiple tabs, and multiplexers inside the tabs). I can’t live without this, and I highly recommend it if you often find yourself hunting around for your terminal window.