I don't know how Firefox could do that. What model of keyboard is it? There probably is some mysterious key combination to re-enable the backlighting.

What a dumb rookie mistake....I was about to give the Logitech part number to you and add the line " I have pushed every button on the top of the keyboard" when I noticed 4 buttons on the far right of the keyboard.. One has a light bulb on it.."Naw..It couldn't be THAT simple !" It was.. I pushed the light bulb key ..and there was light on my keyboard....Now I have to undo all the blasphemy. I can solder and rescue electronic devices and built radio kits in my teens...but a simple button on my computer defeated me.. You were right. It was a simple switch. I must have hit the bulb key while reaching across my desk when plugging in a microphone ... Thank you for making me think in a logical stream of double checking the obvious. All the best, Tom Turner>>

I have a Logitech G410 keyboard and it has two buttons on top right corner. What looks like a light bulb is for turning light on/off. Its annoying that for this KB it is only has the default blue option unless you install the Logitch software for to be able to use others colours.

You may have another key on KB that looks like a joystick as it disables the Windows key (aka Winkey) for when you play games for example so it accidentally triggered.